Used by the routines at TITLE_SCREEN, ROOMST and MAIN_LOOP.

The author's ATTRI: all 768 attribute bytes set to ROOM_COLOUR. A room is drawn unseen, its attributes cleared (DRAW_CURRENT_ROOM), and coloured only when it is complete: under the title and GAME OVER (TITLE_SCREEN), at the end of the quest (ROOMST), and after the first pass of a new room, when every object has been drawn (MAIN_LOOP).