event: lazify screen update

Set root screen change flag in event handler, and update in
draw_callback.

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

src/common.h

@@ -345,6 +345,8 @@ typedef struct session {
 	xcb_sync_fence_t sync_fence;
 
 	// === Operation related ===
+	/// Flags related to the root window
+	uint64_t root_flags;
 	/// Program options.
 	options_t o;
 	/// Whether we have hit unredirection timeout.