Add binding for SDL_mixer Mix_SetPostMix

[LeWolfYT]

I've been working on a project that needs to read from the mixed audio stream to be sent to ffmpeg for streaming. However, the only function I found fit is the Mix_SetPostMix function, which has no analogous function in Python. If some equivalent of this function were implemented, audio visualizations and other similar functions would be possible.

[yunline]

You can try pygame._sdl2.mixer.set_post_mix

[LeWolfYT]

This does work, but I think that it would still be beneficial to move it into the main library for ease of access and documentation purposes, since pygame._sdl2.mixer is undocumented (to my knowledge).