backend/wayland: improve/fix xdg_toplevel (re-)initialization

- Reset all variables representing an initialized xdg_toplevel's state
on unmap.
- Send an initial commit only when an output is about to be enabled.
- If an output isn't configured yet, don't commit a buffer.

include/backend/wayland.h

@@ -99,6 +99,8 @@ struct wlr_wl_output {
 	bool has_configure_serial;
 	bool configured;
 
+	bool initialized;
+
 	uint32_t enter_serial;
 
 	struct {