Return the concatenated string of all object images.

Otherwise they just get dumped but don't make it into the HTML.
Rename objects to interior objects to avoid future confusion.
Remove EXXs in plot_indoor_tiles by renaming registers.

For example, HL -> HL'.
Avoid a goto in queue_message_for_display.
Simplify message_display.
Simplify wipe_message.
Document rotate_A_left_2_widening_to_BC (trivial).
Suspect four_byte_structs are door positions.
Document is_door_open.