Disabled the game using "set background" by making all instances of it use a define

... , which can be changed in code/_compile_options.dm

Testing has revealed that it reduces the sluggishness of the game, though it will slightly spike from lag when the singularity is loose, but will eventually return to normal. Thanks to ChuckTheSheep for suggesting it.

Server owners who want to keep set background enabled can do so by changing the define.