Investigate Providing SoLoud Audio Backend

[Unit158]

Issue Summary

OpenAL has given the OpenRA project grief. SoLoud is a sound engine written in C++ to provide a state-based interface between high level audio operations and low level codec or OS libraries. It provides built-in support for audio queues, bussing and positioning. The only feature that is lacking from SoLoud that is available in OpenAL is the HRTF. There are also built-in solutions for grouping types of sounds, which can be employed to reduce overlap in speech, for example.

I have already made a proof-of-concept implementation, and it only took a few minutes to do. There may be some refactoring of the audio system required before a new back end is made available, but the developers will have a stronger idea of the current state of the plumbing than I.