WAV: support "extensible" format tag.

[icculus]

I put in a quick hack, but here are the details on proper support.

https://docs.microsoft.com/en-us/windows/win32/api/mmreg/ns-mmreg-waveformatextensible

SDL_LoadWAV already has this--at least enough for reasonable use--and I'm aiming for something similar here.