Update SDL Mixer to 2.0.4

Like !321 (merged), straight copypasta of SDL Mixer dev files into libs. MI would like to make sure that nothing breaks in-game.

It builds and plays OK for me.

Download binaries: https://www.libsdl.org/projects/SDL_mixer/release/SDL2_mixer-2.0.4-win32-x86.zip

I think if SDL2 gets updated, this needs to be updated too. I forget if the bundled SDL Mixer with 2.1 fails to play SFX when loaded with a newer SDL version.