xwm: Check size hints in weston_wm_window_is_positioned()

Currently we can't tell the difference between a window intentionally
created at 0,0 and a window that we can place anywhere.

Check the size hints to see if the flags indicating the placement
is intentional are present.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>
(cherry picked from commit 1cb46994e3808e8000300ed9ae9dcaa0b76bff28)

xwayland/window-manager.c

@@ -2937,6 +2937,9 @@ weston_wm_window_is_positioned(struct weston_wm_window *window)
 		weston_log("XWM warning: win %d did not see map request\n",
 			   window->id);
 
+	if (window->size_hints.flags & (USPosition | PPosition))
+		return true;
+
 	return window->map_request_x != 0 || window->map_request_y != 0;
 }