clutter/stage-view: Only blit the damage part of the shadow buffer

This fixes the last "copy everything" paths when clutter doesn't
directly paint onto the onscreen framebuffer. It adds a new hook into
the stage view called before the swap buffer, as at this point, we have
the swap buffer damag regions ready, which corresponds to the regions we
must blit according to the damage reported to clutter.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1237

clutter/clutter/clutter-stage-view-private.h

@@ -23,6 +23,9 @@
 void clutter_stage_view_after_paint (ClutterStageView *view,
                                      cairo_region_t   *redraw_clip);
 
+void clutter_stage_view_before_swap_buffer (ClutterStageView     *view,
+                                            const cairo_region_t *swap_region);
+
 gboolean clutter_stage_view_is_dirty_viewport (ClutterStageView *view);
 
 void clutter_stage_view_invalidate_viewport (ClutterStageView *view);

clutter/clutter/clutter-stage-view.c

@@ -370,11 +370,22 @@ clutter_stage_view_after_paint (ClutterStageView *view,
                                          redraw_clip);
         }
     }
+}
 
-  if (priv->shadow.framebuffer)
+void
+clutter_stage_view_before_swap_buffer (ClutterStageView     *view,
+                                       const cairo_region_t *swap_region)
+{
+  ClutterStageViewPrivate *priv =
+    clutter_stage_view_get_instance_private (view);
+  g_autoptr (GError) error = NULL;
+
+  if (!priv->shadow.framebuffer)
+    return;
+
+  if (cairo_region_is_empty (swap_region))
     {
       int width, height;
-      g_autoptr (GError) error = NULL;
 
       width = cogl_framebuffer_get_width (priv->framebuffer);
       height = cogl_framebuffer_get_height (priv->framebuffer);
@@ -384,12 +395,32 @@ clutter_stage_view_after_paint (ClutterStageView *view,
                                   0, 0,
                                   width, height,
                                   &error))
+        g_warning ("Failed to blit shadow buffer: %s", error->message);
+    }
+  else
+    {
+      int n_rects;
+      int i;
+
+      n_rects = cairo_region_num_rectangles (swap_region);
+      for (i = 0; i < n_rects; i++)
+        {
+          cairo_rectangle_int_t rect;
+
+          cairo_region_get_rectangle (swap_region, i, &rect);
+          if (!cogl_blit_framebuffer (priv->shadow.framebuffer,
+                                      priv->framebuffer,
+                                      rect.x, rect.y,
+                                      rect.x, rect.y,
+                                      rect.width, rect.height,
+                                      &error))
             {
               g_warning ("Failed to blit shadow buffer: %s", error->message);
               return;
             }
         }
     }
+}
 
 float
 clutter_stage_view_get_scale (ClutterStageView *view)

clutter/clutter/cogl/clutter-stage-cogl.c

@@ -379,6 +379,8 @@ swap_framebuffer (ClutterStageWindow *stage_window,
 {
   CoglFramebuffer *framebuffer = clutter_stage_view_get_onscreen (view);
 
+  clutter_stage_view_before_swap_buffer (view, swap_region);
+
   if (cogl_is_onscreen (framebuffer))
     {
       CoglOnscreen *onscreen = COGL_ONSCREEN (framebuffer);