When the fallback for gdk_draw_pixbuf (gdk_drawable_real_draw_pixbuf)
is called with a window destination we have already applied any clip
regions and offsets for the window, but the window we get is a wrapper
and not an impl. We have to ensure we really draw to the impl, as
otherwise the pixbuf drawing will be clipped by client side subwindows.

This fixes bug 588553.