It seems XP doesn't handle drawing non-alpha theme parts
on alpha destinations. We fix this by using alpha bitmaps only when
needed.
However this means any non-drawn area by the theme part is now draw
black, so we must take more care to only draw where the theme part draws,
so we find the theme part size when available.
We now support -gtk-win32-theme-part(class,part,state) in background
and border-image CSS properties. This renders the corresponding
theme part using DrawThemeBackground() and acts as a base for a
CSS based windows theme.
Note that we build the parsing code even on non-win32 so that
all themese will parse the same on all arches. We draw pink instead
of the actual theme parts on non-win32 though.
