Used by the routine at build_screen_objects.

Called before a new room is built (build_screen_objects), once a frame of the game has run (bit 0 of $5BB2). For each of records 2 and 3 that is in use, writes its type and its x, y, z and room back into its entry in special_objs_tbl, through the pointer at +$10. That is how an object pushed or dropped somewhere stays there when you come back.