(cherry picked from commit 9a5eb024)

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

(cherry picked from commit fdbe3e80)

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

(cherry picked from commit b330dc23)

* 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 76be77b9)

(cherry picked from commit cb4e0751)

(cherry picked from commit 0999b0f8)

(cherry picked from commit e72610a3)

(cherry picked from commit 8e05de17)

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

# Conflicts:
#	src/sdl/mixer_sound.c

# Conflicts:
#	src/sdl/mixer_sound.c

Merge branch 'music-cleanup' of https://git.magicalgirl.moe/STJr/SRB2Internal.git into music-cleanup

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

* S_MusicInfo method to retrieve name, flags, and looping

* 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

* 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

* Revised S_PlayMusic arguments
* Now music plays again!

* Pause/ResumeSong remove handle argument
* LoadSong declare char *data argument
* Variable weirdness

This reverts commit 58fbb463.

# Conflicts:
#	src/android/i_sound.c
#	src/dummy/i_sound.c
#	src/win32/win_snd.c

This reverts commit b5c30958.

# Conflicts:
#	src/i_sound.h
#	src/s_sound.c
#	src/s_sound.h
#	src/sdl/mixer_sound.c

* Re-purpose I_LoadSong for digital music loading
* I_StartDigSong logic split between I_LoadSong and I_PlaySong
* Pause, Stop, and Unload routines are combined from Digi and MIDI
* music_lumpnum, music_data, music_handle from s_sound.c are gone