New Audio Mixer

See merge request KartKrew/Kart!847

Libraries superceded by libxmp-lite and new mixer.

This replaces SDL2_mixer.

This is a lightweight single-file Ogg Vorbis decoder which will be used
for Ogg playback instead of libogg/libvorbis.

This is a very lightweight module playback engine which will replace
OpenMPT.

Fix Broly SIGFPE

See merge request KartKrew/Kart!841

Rulesify (resolves #375)

Closes #360 and #375

See merge request KartKrew/Kart!836

Misc. changes for the sake of monitors

See merge request KartKrew/Kart!805

Show CMAKE_BUILD_TYPE on title screen

See merge request KartKrew/Kart!849

File and header fix (resolves #370)

Closes #370

See merge request KartKrew/Kart!848

Don't treat players with 0 lives that are exiting (definitely just completed a race) as irrelevant in P_CheckRacers counts

Fixes special stages not booting you back to the title screen after running out of lives on them

Also clearer variable name for length of cupgrid page in memory

Includes a hint if optimizations may have been turned off.

Add SRB2_ASSERT, srb2::NotNull

See merge request KartKrew/Kart!843

cmake: build discord-rpc internally

See merge request KartKrew/Kart!846

Fix strcasestr redefine warnings for G++

See merge request KartKrew/Kart!844

Was going to `#include "console.h"` but not worth it for literally one placeholder index

# Conflicts:
#	src/k_boss.h
#	src/k_specialstage.h
#	src/y_inter.h

An effectively useless global scope boolean that literally checks for the presence of a finish line, created in and used only by things we inherited from vanilla SRB2