Commit da094479 removed the call to
gtk_style_context_set_background() in favour of always rendering it with
gtk_render_background() during the draw vfunc.
This has the side effect of making the backing window always
transparent, which blocks GTK from applying some optimizations during
the paint cycle. The result is that, especially in clutter-gtk
applications, scrolling performance gets really bad.

This commit partially reverts da094479
and changes the code so that both gtk_style_context_set_background() and
gtk_render_background() are called.