wayland/surface: Unconditionally set scanout destination rect

The cogl_scanout_get_dst_rect() fell back on the buffer dimensions as
the destination rectangle when nothing was explicitly set. This,
however, is not necessarily correct. For example, if a buffer is larger
the CRTC resolution, but the surface is scaled to exactly match the CRTC view,
the expected destination size should match the CRTC resolution, not the
buffer dimension, which would be the case if no explicit destination was
set.

In meta_wayland_try_aquire_scanout() we're in a good position to
determine the destination rect in the CRTC primary plane, since we have
all the prerequisits, i.e. that the surface effectively covers the whole
CRTC, the actor allocation box (the non-black border part), the scale
and transform of the view.

This tweaks the CoglScanout API a bit to make it explicit that the
dst_rect must be unconditionally provided, and removes the fallback to
the buffer dimension as the destination rectangle, which sometimes
resulted in a destination rectangle being larger than the primary plane
itself, resulting in clipping and incorrect scaling.

Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/3773
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4147>

src/wayland/meta-wayland-buffer.c

@@ -924,9 +924,9 @@ try_acquire_egl_image_scanout (MetaWaylandBuffer     *buffer,
       return NULL;
     }
 
-  scanout = cogl_scanout_new (COGL_SCANOUT_BUFFER (g_steal_pointer (&fb)));
+  scanout = cogl_scanout_new (COGL_SCANOUT_BUFFER (g_steal_pointer (&fb)),
+                              dst_rect);
   cogl_scanout_set_src_rect (scanout, src_rect);
-  cogl_scanout_set_dst_rect (scanout, dst_rect);
   if (!meta_onscreen_native_is_buffer_scanout_compatible (onscreen, scanout))
     return NULL;