[metacity/gnome-3-18] frames: use cairo_push_group in meta_frames_get_mask

[Alberts Muktupāvels <muktupavels src gnome org>]

commit 1dfbe40999d9779e27a9810e1594ad79eff938cd
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 1f4b575..497e885 100644
--- a/src/ui/frames.c
+++ b/src/ui/frames.c
@@ -2427,7 +2427,12 @@ 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.