win: delayed release of shadow image

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>

tests/testcases/issue239_3.py

@@ -36,6 +36,8 @@ print("set new name")
 win_name = "NoShadow"
 conn.core.ChangePropertyChecked(xproto.PropMode.Replace, wid, name_atom, str_type_atom, 8, len(win_name), win_name).check()
 
+time.sleep(0.5)
+
 # Set the Window name so it gets a shadow
 print("set new name")
 win_name = "YesShadow"