(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)

New linedef executor special: change tagged sector's colormap

See merge request STJr/SRB2Internal!149

# Conflicts:
#	src/p_setup.c
#	src/r_data.c

Colormap code cleanup

See merge request !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

Netcode nitpicking

See merge request !274

Flicky Thing Types

See merge request STJr/SRB2Internal!154