Update stb.
Big commit improving audio support:

 - Add the beginning of the new AssetLibrary class.
 - Add the beginning of the new SoundWorld class.
 - Add a proper Sound class.
 - Sound streamers now just take a memory pointer and size instead of a file
   path. This makes it easier to work with mapped files.
 - Sounds should now be properly cleaned up when they stop playing

Lots of testing is still needed here. I expect there are a few threading
related bugs hanging around, but I'll get to a better design later - just
focused on getting things working for the moment.