Fix transparency handling with non-double-buffered drawing

Fix transparency handling with non-double-buffered drawing

Sometimes we need to read back the window content into our double
buffer due to rendering a window with alpha when there is
no implicit paint or it has been flushed due to non-db drawing
before.

However, in this case we can't use gdk_cairo_set_source_window as
it might trigger an implicit paint flush as we detect what we
think is a direct non-double buffered window draw operation, which
will flush the implicit paint operation that we're just setting up.

To fix this we use the raw gdk_window_ref_impl_surface operation
to get the source surface.