After introduction of the Idle processing, something has changed so
that the underlying GetGdkWindow() does not update its size fast enough;
even though the gtk_window_resize() is called before the Window::Erase()
(that actually paints the background) etc.

The consequence of the not-yet-updated gdkDrawable is that we cache
something that has the correct background in the top left 200x200
pixels, but the rest is just copied from the screen.

Let's for now just not cache when the GetGdkWindow() is smaller than
what we actually paint; TODO find the root cause.

Change-Id: Ib6092668eb4613899f665bb0f12fc1eb15484e13