backend/x11: only send frames when the window is mapped

Tested with WindowMaker. When switching to another workspace,
minimizing, or rolling up the window, we get an UnmapNotify event, and a
corresponding MapNotify event when the window appears on screen again.

Fixes #2675

include/backend/x11.h

@@ -51,6 +51,7 @@ struct wlr_x11_output {
 	pixman_region32_t exposed;
 
 	uint64_t last_msc;
+	bool mapped;
 
 	struct {
 		struct wlr_swapchain *swapchain;