Drawangle fixes

Closes #153

See merge request STJr/SRB2Internal!236

Prevent just anyone from skipping credits but allow admins

See merge request STJr/SRB2!478

Pk3 Leaks

See merge request !500

ACZ hardcoding (including minecarts)

See merge request STJr/SRB2Internal!231

Stand(alone) lump comedy

See merge request !499

FOF slopes level load fix

See merge request STJr/SRB2!498

libopenmpt support

See merge request STJr/SRB2Internal!229

Fix warnings/errors found during compiling, mostly of the mixed-declaration-and-code or shadowed variables variety

Part 2 of fix, make sure copied slopes also pass on hasslope status to attached target sectors for FOFs

Also fix whitespace to use tab-style spaces instead of regular spaces, ew

Fix FOF slopes briefly glitching on level load in software mode, by ensuring the FOFs' target sectors have hasslope set on creation

(Hopefully) fixed the player sometimes landing on the solid part of the minecart stopper after being launched out of the minecart