waypipe now records all wl_surface.damage requests made to a given surface,
and at surface commit time, determines which range of bytes in the attached
wl_buffer's memory pool file may have changed. This range can be much
smaller than the size of the file, so that change detection on the range
can be significantly cheaper than full-file change detection.

This commit also sets up subtyping for struct wp_object, to reduce the memory
usage needed for typical objects.