Also don't change mask corner images when color theme is changed.
This prevents race condition in mask corner images access, because
the GIF frame readers access mask corner images from other threads.
Currently the build without implicitly included precompiled header
is not supported anyway (because Qt MOC source files do not include
stdafx.h, they include plain headers).
So when we decide to support building without implicitly included
precompiled headers we'll have to fix all the headers anyway.
