Used by the routine at end_of_frame.

Called at the end of every frame. The list at objects_to_draw holds every object that is to be drawn this frame: those that moved or changed, which set_wipe_and_draw_flags flagged with bits 4 and 5 of byte 7, and those whose picture overlaps them, which set_draw_objs_overlapped flagged with bit 4 only. First, for each object that moved, the rectangle covering both where it was last frame and where it is now is cleared in the buffer and remembered on the stack. Then every flagged object is drawn into the buffer in depth order, and finally the remembered rectangles -- and only those -- are copied to the display.

Nothing is ever erased on the display itself, so nothing flickers; and since the background is made of objects too, a wall behind a moving knight is simply drawn again.

In the frame a room is entered the wiping is skipped altogether: the buffer has just been cleared, and end_of_frame copies all of it to the display with update_screen once the objects are drawn.