Music Code Cleanup

See merge request !278

Handle fix for SDL Mixer

See merge request !301

More accurate GIF delay

See merge request !276

MI's unimportant code cleanup

See merge request !295

I've voided this out on other sound interfaces than SDL Mixer ones because I'm both not sure whether they need it, and not sure how to make them work with it if they do.

Remove "playerdeadview" variable; it's not been used for its stated purpose for who knows how long now

Besides rankings popping up when you die just sounds weird anyway, maybe I'm just used to SRB2 not doing it I guess

Remove unused ObjectPlace_OnChange prototype (from when Objectplace was a consvar, which it is not anymore)

Also corrected what appears to be a typo in some comments above?

(it's only set to 0 in software, and only ++'d in OpenGL, what kind of sense does that make?)

Tiny fix so that joystick2 being closed can let the JoystickSubSystem close before game close.

See merge request !292

No memory leak here, just a very tiny thing I noticed.

Free SOC_s to a good home

See merge request !291

* Fix a memory leak regarding implementation of SOC_ (improperly copypasted code from LUA_LoadLump!!)
* Optimise the repeated strlen usage into a single call, which is stored for later.

Dr. Who And The Sleeping Weather

See merge request !288

SP savegame continues icon crash

See merge request !290

Makefile: Split zlib and libpng

See merge request !285

Fixer mixer: Mixer Memory Leaks

See merge request !287

Weather is already run client-side. What if we ran it render-side, for major performance gains? This commit will answer all your questions - and more!

A value of 1 in freesrc for Mix_LoadWAV_RW and Mix_LoadMus_RW calls SDL_RWclose on the RWops anyway.
For Mix_LoadWAV_RW the RWops is freed right after the data is loaded (because it makes a copy of the data in memory)
For Mix_LoadMUS_RW the RWops is freed when Mix_FreeMusic is called (because the data is not a copy)
So setting 1 on freesrc doesn't actually free the RWops immediately on Mix_LoadMus_RW *unless* it failed to load any music.

Mix_QuickLoad_RAW sets a flag in the Mix_Chunk so that Mix_FreeChunk doesn't actually Free the sound.
Checks for the flag when freeing, and if it's 0, we free the data manually after Mix_FreeChunk.
I went back to Z_Malloc and Z_Free for this because they still work after this.

Use MemAvailable instead

See merge request !281

Thanks again MonsterIestyn!