layer_shell: damage previous area when a surface shrinks

When a layer surface shrinks we need to damage the area it previously
occupied, but we don't know the location of all its subsurfaces in the
previous state, so instead damage a rectangle that encloses the entire
previous extent.

include/sway/layers.h

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