core: Add infrastructure to keep window frames' opaque regions

These frames client will use a visual with alpha information, and
report the opaque frame shapes through the _NET_WM_OPAQUE_REGION
window property. We can use this information in the Mutter side
for accurate opaque shapes, despite X11 windows with frames now
being seen as possibly transparent.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2758>

src/core/frame.c

@@ -26,6 +26,7 @@
 #include "core/frame.h"
 
 #include "backends/x11/meta-backend-x11.h"
+#include "compositor/compositor-private.h"
 #include "core/bell.h"
 #include "core/keybindings-private.h"
 #include "meta/meta-x11-errors.h"
@@ -232,6 +233,8 @@ meta_window_destroy_frame (MetaWindow *window)
       window->frame_bounds = NULL;
     }
 
+  g_clear_pointer (&window->opaque_region, cairo_region_destroy);
+
   /* Move keybindings to window instead of frame */
   meta_window_grab_keys (window);
 
@@ -600,3 +603,20 @@ meta_frame_get_sync_counter (MetaFrame *frame)
 {
   return &frame->sync_counter;
 }
+
+void
+meta_frame_set_opaque_region (MetaFrame      *frame,
+                              cairo_region_t *region)
+{
+  MetaWindow *window = frame->window;
+
+  if (cairo_region_equal (frame->opaque_region, region))
+    return;
+
+  g_clear_pointer (&frame->opaque_region, cairo_region_destroy);
+
+  if (region != NULL)
+    frame->opaque_region = cairo_region_reference (region);
+
+  meta_compositor_window_shape_changed (window->display->compositor, window);
+}

src/core/frame.h

@@ -41,6 +41,8 @@ struct _MetaFrame
 
   MetaFrameBorders cached_borders; /* valid if borders_cached is set */
 
+  cairo_region_t *opaque_region;
+
   MetaSyncCounter sync_counter;
 
   /* position of client, size of frame */
@@ -81,4 +83,7 @@ GSubprocess * meta_frame_launch_client (MetaX11Display *x11_display,
 
 MetaSyncCounter * meta_frame_get_sync_counter (MetaFrame *frame);
 
+void meta_frame_set_opaque_region (MetaFrame      *frame,
+                                   cairo_region_t *region);
+
 #endif

src/x11/window-props.c

@@ -710,7 +710,11 @@ reload_opaque_region (MetaWindow    *window,
     }
 
  out:
+  if (value->source_xwindow == window->xwindow)
     meta_window_set_opaque_region (window, opaque_region);
+  else if (window->frame && value->source_xwindow == window->frame->xwindow)
+    meta_frame_set_opaque_region (window->frame, opaque_region);
+
   cairo_region_destroy (opaque_region);
 }