x11: Simplify drag cancel animation setup

x11: Simplify drag cancel animation setup

Instead of creating an intermediate pixbuf, just render
the window surface onto the new surface. Doing things this
way lets us avoid the cairo_surface_mark_dirty() call in
gdk_pixbuf_get_from_window(), which is not generally safe
to call on 'random' surfaces - it asserts that the surface
has no mime data attached, and the X11 backend uses mime
data for damage tracking purposes...