(cherry picked from commit 5b724e18)

* 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 5f21bf23)

(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 a7d51bf8)

* S_MusicInfo method to retrieve name, flags, and looping

(cherry picked from commit f6ec9319)

(cherry picked from commit 4aa100aa)

(cherry picked from commit 86f151db)

(cherry picked from commit 07738fb0)

(cherry picked from commit d5ec3881)

* 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 f5f0b5e7)

(cherry picked from commit 9fb9386f)

* 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 701cc5a7)

(cherry picked from commit 14b393ab)

* Revised S_PlayMusic arguments
* Now music plays again!

(cherry picked from commit 55f3803e)

(cherry picked from commit 9a5fc5f66a31baf713c6f32f7a696b933da6907b)

Colormap code cleanup

See merge request STJr/SRB2!275

R_CreateColormap2 and R_MakeColormaps have been made obsolete, it's just R_CreateColormap now, like it used to be!

With that, I moved R_CreateColormap2's exclusive software colormap malloc code to R_CreateColormap, and merged the two software-only blocks of code into one. I also disabled any unneeded variables and fixed a preprocessor-related goofup