sokol_audio.h: make CoInitializeEx call configurable.

[floooh]

Should be a new item in saudio_desc, and allow three behaviours:

• call CoInitializeEx with COINIT_MULTITHREADED (the default)
• call CoInitializeEx with COINIT_APARTMENTTHREADED
• don't call CoInitializeEx (e.g. must be called outside sokol-audio)

Also see: https://github.com/floooh/sokol/issues/1398