Reimplement prefoppositecolor's Functionality

Exactly what it says on the tin. prefoppositecolor is a skin variable that exists in both vanilla SRB2 and Ring Racers whose intended purpose is to change the background color used for the signpost whenever said skin is using its prefcolor. However, it actually ends up completely unused in Ring Racers for whatever reason, so this merge request restores its SRB2 behavior.