compositor: introduce surface state squashing

State squashing refers to the action of combining the state with the one
right before it; the older state becomes a "sum" of two states, and the
newer state becomes empty. This allows to free up memory and release
client resources as soon as possible.

include/wlr/types/wlr_compositor.h

@@ -165,6 +165,7 @@ struct wlr_surface {
 		enum wl_output_transform transform;
 		int width, height;
 		int buffer_width, buffer_height;
+		struct wlr_fbox viewport_src;
 	} previous;
 };