[mutter] background-content: Apply Cullable clipping even in the transformed case

[Robert Mader <rmader src gnome org>]

commit 64d34a76489ce46926ece828259ed4c394730f94
Author: Daniel van Vugt <daniel van vugt canonical com>
Date:   Thu Oct 1 18:01:46 2020 +0800

    background-content: Apply Cullable clipping even in the transformed case
    
    Just like we used to before 30809665d8.
    
    Because in some cases `clip_region` is able to shave off an extra pixel
    from the edge of the redraw rectangle(s). And not shaving that off was
    making the background rendering inconsistent with shaped-texture, causing
    occasional off-by-one artefacts. Now both shaped-texture and
    background-content agree on the clip region again that doesn't happen.
    
    Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1443
    
    https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1464

 src/compositor/meta-background-content.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)
---
diff --git a/src/compositor/meta-background-content.c b/src/compositor/meta-background-content.c
index bf203aa9af..ce6ee2fff1 100644
--- a/src/compositor/meta-background-content.c
+++ b/src/compositor/meta-background-content.c
@@ -557,15 +557,23 @@ meta_background_content_paint_content (ClutterContent      *content,
               region = cairo_region_create_rectangle (&rect_within_stage);
             }
         }
-
-      if (self->unobscured_region)
-        cairo_region_intersect (region, self->unobscured_region);
     }
   else /* actor and stage space are different but we need actor space */
     {
-      region = cairo_region_create_rectangle (&rect_within_actor);
+      if (self->clip_region)
+        {
+          region = cairo_region_copy (self->clip_region);
+          cairo_region_intersect_rectangle (region, &rect_within_actor);
+        }
+      else
+        {
+          region = cairo_region_create_rectangle (&rect_within_actor);
+        }
     }
 
+  if (self->unobscured_region)
+    cairo_region_intersect (region, self->unobscured_region);
+
   /* region is now in actor space */
   if (cairo_region_is_empty (region))
     {