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"

tests/testcases/issue239_3_norefresh.py

@@ -0,0 +1,52 @@
+#!/usr/bin/env python
+
+import xcffib.xproto as xproto
+import xcffib
+import time
+
+conn = xcffib.connect()
+setup = conn.get_setup()
+root = setup.roots[0].root
+visual = setup.roots[0].root_visual
+depth = setup.roots[0].root_depth
+
+# issue 239 is caused by a window gaining a shadow during its fade-out transition
+wid = conn.generate_id()
+print("Window id is ", hex(wid))
+
+# Create a window
+conn.core.CreateWindowChecked(depth, wid, root, 0, 0, 100, 100, 0, xproto.WindowClass.InputOutput, visual, 0, []).check()
+
+# Set Window name so it doesn't get a shadow
+name = "_NET_WM_NAME"
+name_atom = conn.core.InternAtom(True, len(name), name).reply().atom
+str_type = "STRING"
+str_type_atom = conn.core.InternAtom(True, len(str_type), str_type).reply().atom
+
+win_name = "YesShadow"
+conn.core.ChangePropertyChecked(xproto.PropMode.Replace, wid, name_atom, str_type_atom, 8, len(win_name), win_name).check()
+
+# Map the window
+print("mapping")
+conn.core.MapWindowChecked(wid).check()
+
+time.sleep(0.5)
+
+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()
+
+# Set the Window name so it gets a shadow
+print("set new name")
+win_name = "YesShadow"
+conn.core.ChangePropertyChecked(xproto.PropMode.Replace, wid, name_atom, str_type_atom, 8, len(win_name), win_name).check()
+
+time.sleep(0.5)
+
+# Unmap the window
+conn.core.UnmapWindowChecked(wid).check()
+
+time.sleep(0.5)
+
+# Destroy the window
+conn.core.DestroyWindowChecked(wid).check()