Move texture hacks into a GameModeInfo callback. Eliminates last substantial branch on mission info ids (only remaining one is for Doom 2 localized versions, which is OK how it is since it will only ever apply to doom2 mission).

git-svn-id: http://mancubus.net/svn/hosted/eternity/trunk@2875 86ba831e-150c-0410-ab35-e06eaae15ed9