compositor: Expose the MetaCompositorView in before_paint () vfunc

The idea is that the state of the MetaCompositorView shall be
up-to-date only in specific scenarios, thus allowing operations
performed on it to be queued and aggregated to be handled in the
right time, and only if they are still necessary.

For example, in a following commit, the top window actor in each
view will be planned (if needed) only once before painting a frame,
rendering the top window actor in the MetaCompositorView potentially
stale in all other times.

Similarly, if a MetaCompositorView is destroyed before the beginning
of the frame, a queued operation to update its top window actor can be
discarded.

As an interface segragation measure, and as part of an attempt to
avoid the use of g_return_if_fail () to check the validity of the
MetaCompositorView's state in multiple places (which is still prone to
human error), the interfaces through which a MetaCompositorView is
made available would only ones where it's state is gurenteed to be
up-to-date.

Specifically, this commit gurentees that the state of the
MetaCompositorView would be up-to-date during the before_paint () and
after_paint () vfuncs exposed to child classes of the MetaCompositor.

The frame_in_progress variable will be used in a following commit to
guarantee that the MetaCompositorView's state is not invalidated during
this time.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2526>

src/compositor/compositor-private.h

@@ -7,6 +7,7 @@
 
 #include "clutter/clutter-mutter.h"
 #include "clutter/clutter.h"
+#include "compositor/meta-compositor-view.h"
 #include "compositor/meta-plugin-manager.h"
 #include "compositor/meta-window-actor-private.h"
 #include "meta/compositor.h"
@@ -25,9 +26,9 @@ struct _MetaCompositorClass
                        GError         **error);
   void (* unmanage) (MetaCompositor *compositor);
   void (* before_paint) (MetaCompositor     *compositor,
-                         ClutterStageView *stage_view);
+                         MetaCompositorView *compositor_view);
   void (* after_paint) (MetaCompositor     *compositor,
-                        ClutterStageView *stage_view);
+                        MetaCompositorView *compositor_view);
   void (* remove_window) (MetaCompositor *compositor,
                           MetaWindow     *window);
   int64_t (* monotonic_to_high_res_xserver_time) (MetaCompositor *compositor,

src/compositor/compositor.c

@@ -61,7 +61,6 @@
 #include "backends/x11/meta-stage-x11.h"
 #include "clutter/clutter-mutter.h"
 #include "cogl/cogl.h"
-#include "compositor/meta-compositor-view.h"
 #include "compositor/meta-later-private.h"
 #include "compositor/meta-window-actor-x11.h"
 #include "compositor/meta-window-actor-private.h"
@@ -131,6 +130,8 @@ typedef struct _MetaCompositorPrivate
 
   int switch_workspace_in_progress;
 
+  gboolean frame_in_progress;
+
   MetaPluginManager *plugin_mgr;
 
   MetaLaters *laters;
@@ -987,33 +988,43 @@ on_presented (ClutterStage     *stage,
 
 static void
 meta_compositor_real_before_paint (MetaCompositor     *compositor,
-                                   ClutterStageView *stage_view)
+                                   MetaCompositorView *compositor_view)
 {
   MetaCompositorPrivate *priv =
     meta_compositor_get_instance_private (compositor);
+  ClutterStageView *stage_view;
   GList *l;
 
+  stage_view = meta_compositor_view_get_stage_view (compositor_view);
+
   for (l = priv->windows; l; l = l->next)
     meta_window_actor_before_paint (l->data, stage_view);
 }
 
 static void
 meta_compositor_before_paint (MetaCompositor     *compositor,
-                              ClutterStageView *stage_view)
+                              MetaCompositorView *compositor_view)
 {
+  MetaCompositorPrivate *priv =
+    meta_compositor_get_instance_private (compositor);
+
   COGL_TRACE_BEGIN_SCOPED (MetaCompositorPrePaint,
                            "Compositor (before-paint)");
-  META_COMPOSITOR_GET_CLASS (compositor)->before_paint (compositor, stage_view);
+
+  priv->frame_in_progress = TRUE;
+
+  META_COMPOSITOR_GET_CLASS (compositor)->before_paint (compositor, compositor_view);
 }
 
 static void
 meta_compositor_real_after_paint (MetaCompositor     *compositor,
-                                  ClutterStageView *stage_view)
+                                  MetaCompositorView *compositor_view)
 {
   MetaCompositorPrivate *priv =
     meta_compositor_get_instance_private (compositor);
   ClutterActor *stage_actor = meta_backend_get_stage (priv->backend);
   CoglGraphicsResetStatus status;
+  ClutterStageView *stage_view;
   GList *l;
 
   status = cogl_get_graphics_reset_status (priv->context);
@@ -1040,6 +1051,8 @@ meta_compositor_real_after_paint (MetaCompositor   *compositor,
       break;
     }
 
+  stage_view = meta_compositor_view_get_stage_view (compositor_view);
+
   for (l = priv->windows; l; l = l->next)
     {
       ClutterActor *actor = l->data;
@@ -1053,11 +1066,16 @@ meta_compositor_real_after_paint (MetaCompositor   *compositor,
 
 static void
 meta_compositor_after_paint (MetaCompositor     *compositor,
-                             ClutterStageView *stage_view)
+                             MetaCompositorView *compositor_view)
 {
+  MetaCompositorPrivate *priv =
+    meta_compositor_get_instance_private (compositor);
+
   COGL_TRACE_BEGIN_SCOPED (MetaCompositorPostPaint,
                            "Compositor (after-paint)");
-  META_COMPOSITOR_GET_CLASS (compositor)->after_paint (compositor, stage_view);
+  META_COMPOSITOR_GET_CLASS (compositor)->after_paint (compositor, compositor_view);
+
+  priv->frame_in_progress = FALSE;
 }
 
 static void
@@ -1065,7 +1083,14 @@ on_before_paint (ClutterStage     *stage,
                  ClutterStageView *stage_view,
                  MetaCompositor   *compositor)
 {
-  meta_compositor_before_paint (compositor, stage_view);
+  MetaCompositorView *compositor_view;
+
+  compositor_view = g_object_get_qdata (G_OBJECT (stage_view),
+                                        quark_compositor_view);
+
+  g_assert (compositor_view != NULL);
+
+  meta_compositor_before_paint (compositor, compositor_view);
 }
 
 static void
@@ -1073,7 +1098,14 @@ on_after_paint (ClutterStage     *stage,
                 ClutterStageView *stage_view,
                 MetaCompositor   *compositor)
 {
-  meta_compositor_after_paint (compositor, stage_view);
+  MetaCompositorView *compositor_view;
+
+  compositor_view = g_object_get_qdata (G_OBJECT (stage_view),
+                                        quark_compositor_view);
+
+  g_assert (compositor_view != NULL);
+
+  meta_compositor_after_paint (compositor, compositor_view);
 }
 
 static void

src/compositor/meta-compositor-native.c

@@ -157,7 +157,7 @@ done:
 
 static void
 meta_compositor_native_before_paint (MetaCompositor     *compositor,
-                                     ClutterStageView *stage_view)
+                                     MetaCompositorView *compositor_view)
 {
   MetaCompositorClass *parent_class;
 
@@ -166,7 +166,7 @@ meta_compositor_native_before_paint (MetaCompositor   *compositor,
 #endif
 
   parent_class = META_COMPOSITOR_CLASS (meta_compositor_native_parent_class);
-  parent_class->before_paint (compositor, stage_view);
+  parent_class->before_paint (compositor, compositor_view);
 }
 
 MetaCompositorNative *

src/compositor/meta-compositor-x11.c

@@ -386,7 +386,7 @@ on_after_update (ClutterStage     *stage,
 
 static void
 meta_compositor_x11_before_paint (MetaCompositor     *compositor,
-                                  ClutterStageView *stage_view)
+                                  MetaCompositorView *compositor_view)
 {
   MetaCompositorX11 *compositor_x11 = META_COMPOSITOR_X11 (compositor);
   MetaCompositorClass *parent_class;
@@ -394,7 +394,7 @@ meta_compositor_x11_before_paint (MetaCompositor   *compositor,
   maybe_unredirect_top_window (compositor_x11);
 
   parent_class = META_COMPOSITOR_CLASS (meta_compositor_x11_parent_class);
-  parent_class->before_paint (compositor, stage_view);
+  parent_class->before_paint (compositor, compositor_view);
 }
 
 static void