layer-shell: fix commit handler

This commit makes sure the extents are kept up-to-date, fixes not
damaging the surface if its layer shell-specific state didn't change,
and adds a check if the layer shell-specific state didn't change but the
surface got mapped/unmapped, which could affect keyboard focus.

include/sway/layers.h

@@ -22,6 +22,7 @@ struct sway_layer_surface {
 	struct wl_listener new_subsurface;
 
 	struct wlr_box geo;
+	bool mapped;
 	struct wlr_box extent;
 	enum zwlr_layer_shell_v1_layer layer;
 };