Used by the routines at no_delay, print_charms_and_show and display_text_list.

Used when a new room is shown: the whole room is composed in screen_buffer first and then appears at once. The buffer runs the other way up from the display -- its first row is the bottom line of the screen -- because the game's pixel y counts up from the bottom; so the copy starts at the display's bottom-left byte, $57E0, and works up a line at a time.

Clearing as it copies leaves the buffer blank. That is safe because from here on only the areas render_dynamic_objects wipes and redraws are ever copied out of it, so what the rest of the buffer holds does not matter.