win: split struct win

Currently compton handles window creation event by immediately query all
the information it needs and create a window struct with that
information. However, this is prone to race conditions.

In the future, we want to react to window creation event by creating a
placeholder in the window stack, and only query window information in a
critical section where the X server is grabbed by us.

This commit split struct win into two struct, one as placeholder, the
other for holding actual window information.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/opengl.c

@@ -221,7 +221,7 @@ void glx_destroy(session_t *ps) {
 		return;
 
 	// Free all GLX resources of windows
-	list_foreach(win, w, &ps->window_stack, stack_neighbour) {
+	win_stack_foreach_managed(w, &ps->window_stack) {
 		free_win_res_glx(ps, w);
 	}