Ogg Vorbis loading/streaming

[olofson]

Add A2S support for loading Ogg Vorbis files as waves.

We probably actually just want to load the encoded data, and then decode from memory on the fly, to avoid using massive amounts of RAM for decoded data. This still needs to work with wtosc, so a new internal wave type and some refactoring will be needed.

Use stb_vorbis for decoding...? If this is going to run in the audio context, I really don't want to trust any external libraries with it! (Probably not even if it runs in a background thread - but that's not an option anyway if seeking via 'phase' is to be implemented.)

(See also #76 and #96.)