[compiz] Gwd rendering errors with metacity_theme_opacity = 1, maximized, use_metacity_theme

[Andrea Cimitan]

I had tried to have a closer look at the source code, to see if there's
a leak somewhere... but haven't find anything where I can help.
The bug, reproduceable since April, is better described here.
https://bugs.freedesktop.org/show_bug.cgi?id=12450

- Andrea