This variable array returns whether any one of the 8 backgrounds
(numbered form 0 - 7 within the array) of the current room is
visible. It can be set to true or false to toggle
the visibility of the selected background in the room too.
if !background_visible false background_visible = true;
The above code checks the current visibility of the background0 in the current room and if it is not visible, it changes the flag to true, thus making it visible.