stylecontext: Fix gtk_style_context_set_background()

stylecontext: Fix gtk_style_context_set_background()

This function is just a sophisitcated optimization.

If we know the GDK window's background will be opaque, we mark it as
opaque. This is so GDK can do all the optimizations it does for opaque
windows and be fast.

This is mainly used when scrolling.

The previous code didn't get this right, in particular it didn't enforce
a transparent background when it knew the background was not opaque.