Sometimes, there needs to be code that gets ran at the end of the game
loop, otherwise rendering issues might occur. Currently, we do this by
special-casing each deferred routine (e.g. shouldreturntoeditor), but it
would be better if we could generalize this deference instead.
Deferred callbacks can be added using the DEFER_CALLBACK macro. It takes
in one argument, which is the name of a function, and that function must
be a void function that takes in no arguments. Also, due to annoying C++
quirks, void functions taking no arguments cannot be attributes of
objects (because they have an implicit `this` parameter), so it's
recommended to create each callback separately before using the
DEFER_CALLBACK macro.
