Re-acquire root pixmap after root reconfiguration

In configure_root, the root pixmap is freed by destroy_backend, and is
never acquired again.

Usually this doesn't seem to cause any problems, maybe because we
immediately receives a root damage notification. But under rare
circumstances we ended up not showing the wallpaper.

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

src/picom.c

@@ -848,6 +848,9 @@ void configure_root(session_t *ps, int width, int height) {
 				ev_break(ps->loop, EVBREAK_ALL);
 				return;
 			}
+
+			// Re-acquire the root pixmap.
+			root_damaged(ps);
 		}
 		force_repaint(ps);
 	}