Previously win_set_shadow tries to release the shadow image when turning
off shadow for a window. When shadow is turned off _immediately_ after
it's turned on, picom won't have a chance to handle the delayed creation
of the shadow before win_set_shadow tries to release the shadow image,
causing a assertion failure because win_set_shadow tried to release a
non-existing image.
This commit makes releasing the shadow image delayed as well.
In theory, we could check the STALE flag in win_set_shadow before
release the image, but that duplicates the logic that is already in
win_process_flags.
Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>
