[102] Blackout Curtains

So kmixerAsync (and by extension kmixer) is more or less done. I just need to add some small interface stuff and fix a bug or two.

Here's a demo of it working:

Here's the code to that demo:

And here's a list of every (public) function that's currently available:

(word wrap kinda butchers this but whatever, you get the idea that it's a lot)

Here's the actual code of Async:

Here's the code size of everything thus far.

I should probably be able to get working on images and video stuff next, which shouldn't take nearly as long, since SDL supports that stuff a lot more than it does audio.