gdk: Rewrite background handling

gdk: Rewrite background handling

Now the window background is a cairo_pattern_t. The backends will try to
set this as good as they can on the windowing system, but no guarantees
are made on wether the windowing system supports the pattern.

Also gets rid of GDK_NO_BG as undefined behavior is not a good idea to
support, and GDK_NO_BG effectively made the window's contents undefined.
It wasn't effectively used in GTK anyway.