frames: "Initialize" frame background

Frames are painted on the frame window according to the GTK+ theme.
Depending on the target's visual, this means either drawing over
a black destination or a fully transparent one. So in cases where
the theme doesn't paint decorations with full opacity, decorations
for windows with an rgba visual look different from those with a
non-rgba visual. Using an rgba visual for all frames independent
from the client's visual can potentially break clients, so our
only option for a consistent appearance is to explicitly initialize
the frame background to black before painting the theme's decoration
on top.

https://bugzilla.gnome.org/show_bug.cgi?id=745060

src/ui/frames.c

@@ -1403,6 +1403,13 @@ meta_frames_draw (GtkWidget *widget,
   gdk_cairo_region (cr, region);
   cairo_clip (cr);
 
+  /* The target may be cleared to black or transparent, depending
+   * on the frame's visual; we don't want decorations to appear
+   * differently when the theme's decorations aren't fully opaque,
+   * so clear to black first
+   */
+  cairo_paint (cr);
+
   meta_ui_frame_paint (frame, cr);
   cairo_region_destroy (region);