We really wouldn't have wanted to start writing inline ASM in the
middle of a conditional expression just to get that janky `CMP AX, 0`
instead of TCC's sensible `OR AX, AX` optimization.
Part of P0146, funded by -Tom- and Ember2528.
Might as well cover this one while I've got this type of function in my
head. This one is used for the animated star backgrounds during Stage 2
and the Shinki battle.
Completes P0073, funded by [Anonymous] and -Tom-.