Split portal-related code to its own source files.
Most of the 2-line-specific setup has been moved to the function which adds a 2-line case. The portals should render as they used to so far, anyway.

Hardcode some ACZ things

See merge request STJr/SRB2Internal!220

Fix SIGSEGV from sending lindefs/sidedefs in savegame due to duplicated lines.

See merge request STJr/SRB2!490

Set modifiers in a non-reactive manner

See merge request STJr/SRB2!456

Don't skip frames when connecting or paused

See merge request STJr/SRB2!489

This reverts commit 7ac0a8b4.

Alias recursion hotfix 2

See merge request !482

Fix music load lag by reverting tag changes

See merge request STJr/SRB2!486

Merge in next and don't billboard papersprites in GL

See merge request !488

# Conflicts:
#	src/p_user.c

Ban IP command

See merge request STJr/SRB2!460

Don't count down wait timer when executing a command line

See merge request !483

OpenGL Sprite Billboarding

See merge request !485

add parentheses around these conditions to fix sloped lights cutting out lights on FOF walls for apparently no reason

not making a merge request because this is so laughably stupidly simple

Don't enable UPnP support if NONET is defined.

See merge request !484

New title screen features

See merge request STJr/SRB2Internal!204

Because songs without this tag will skiplag when loading.

1. It's slow, 2. It's incorrect

Should work on both normal and precipitation sprites.
Sprites that are split by lighting should work correctly.

Further Mace Advancements

See merge request STJr/SRB2Internal!147

Translucent FOF plane fullbright flip

See merge request STJr/SRB2Internal!218