Fix flashing in non-double-buffered widgets

Fix flashing in non-double-buffered widgets

Avoid copying back partially drawn double-buffer data
when flushing to avoid flicker. This means non double
buffered widgets must draw opaque pixels in its expose
handlers, and that you are not allowed to use direct
rendering (or modify GdkWindow pos/size/order) from
inside the expose handler of a double buffered widget.

See https://bugzilla.gnome.org/show_bug.cgi?id=679144 for more
details