(cherry picked from commit 5b724e18b5268b0492b3f8b8980aadd2dd41e235)

* Check SDL Mixer 2.0.3 for MUS_MODPLUG_UNUSED, MUS_MP3_MAD_UNUSED
* Mixed D+C in I_LoadSong

* Mixer: make I_StartupSound return early if already set up
* Restartaudio: Add StopSFX call

* Mixer code has been in mixer_sound.c; this file is not invoked unless compiling with NOMIXER=1
* Remove everything under #ifdef HAVE_MIXER because this is never triggered
* Comment out #ifdef HAVE_LIBGME because we don't support playing music anyway (but theoretically, it could have worked separately from Mixer)
* Stub new music calls

(cherry picked from commit a7ae0599)

(cherry picked from commit e88d1477)

* Stop-gap for now. Ideally the logic would be in the respective places.

# Conflicts:
#	src/sdl/mixer_sound.c

(cherry picked from commit eae5d333)

# Conflicts:
#	src/sdl/mixer_sound.c

(cherry picked from commit d39b7011)

(cherry picked from commit fb6f8c8a)

(cherry picked from commit 5f21bf230d9716b7cc5b52e4bf3591ebe01e9f3a)

(cherry picked from commit 9e6eebeb)

(cherry picked from commit 4b82de9e)

(cherry picked from commit cf065e10)

* I_GetMusicType() -> I_MusicType()
* Wrap MIDI volume hack in #ifdef _WIN32

(cherry picked from commit a7d51bf81030c228937a8e759f8f43b85817fce6)

* S_MusicInfo method to retrieve name, flags, and looping

(cherry picked from commit f6ec93198f0dcfa1d053cca88172c3e3c7ba310c)

(cherry picked from commit 4aa100aa575cc7fc14a743085222c806ba2c714a)

(cherry picked from commit 86f151db)

(cherry picked from commit 07738fb0bcf640a4349337373f451fe68e024b2a)

(cherry picked from commit d5ec38815968e267aceb59a48a6cb6d3292c0b69)

* S_Init -> S_InitSfxChannels because it did mostly SFX anyway
* S_MusicPlaying, S_MusicPaused, S_MusicName, S_MusicExists new status methods
* I_MusicPlaying, I_MusicPaused

(cherry picked from commit f5f0b5e76c2fd405c8cc895dde653c5ed2652622)

(cherry picked from commit 9fb9386f84b3739fe765a78bcb9683eb7e98bc36)

* In s_sound, they are merged to one method as well, but there are still two separate digvolume and seqvolume variables
* Simplified Dig/MidiMusicDisabled in s_sound
* Method reordering

(cherry picked from commit 701cc5a7dd1dfead87a42ec7558c9fa6a1deb193)

(cherry picked from commit 14b393ab16736bb44dab6fd4a90b7bdd8ff782e0)

* Revised S_PlayMusic arguments
* Now music plays again!

(cherry picked from commit 55f3803e4b9f8104c90cc6c769d54121e5bac0b8)

(cherry picked from commit 9a5fc5f66a31baf713c6f32f7a696b933da6907b)

Colormap code cleanup

See merge request STJr/SRB2!275