All remaining users of that vfunc now implement snapshot using cairo
render nodes (win32 and radial).
Also, GtkCssImageClass.snapshot is now NULL, so if a subclass doesn't
implement it, it will now crash.
Previously it would try to call the draw vfunc.
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.
