[metacity] frames: use cairo_push_group in meta_frames_get_mask

[Alberts Muktupāvels <muktupavels src gnome org>]

commit d404e81f3c4bd235cd81d553de784ec4cf6580bb
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Tue Aug 9 22:41:45 2016 +0300

    frames: use cairo_push_group in meta_frames_get_mask

 src/ui/frames.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/src/ui/frames.c b/src/ui/frames.c
index 7af9a4f..2e75caa 100644
--- a/src/ui/frames.c
+++ b/src/ui/frames.c
@@ -2184,7 +2184,12 @@ meta_frames_get_mask (MetaFrames *frames,
   if (frame == NULL)
     return;
 
+  cairo_push_group (cr);
+
   meta_frames_paint (frames, frame, cr);
+
+  cairo_pop_group_to_source (cr);
+  cairo_paint (cr);
 }
 
 /* XXX -- this is disgusting. Find a better approach here.