Handle surround sound-encoded OGG and FLAC files more gracefully

[SirYodaJedi]

BLooperZ (rzil on the forums) is ​currently making scripts that convert the audio from the Full Throttle into a format compatible with ScummVM. One notable hurdle with this is that the cutscene audio is encoded as 5.1 alternative (2 Fronts, Center, LFE, Rear) Ogg Vorbis. When placed as-is next to a zlib-compressed .san file, ScummVM simply treats it as mute (or ignores it?). Whilst this is easily rectified by downmixing to 2.0, I see no reason why this cannot be handled by ScummVM itself, as it is already a pre-mixed audio stream.

What I am proposing is that ScummVM allow up to 8-channel surround sound (including the optional layout metadata) and either output to the audio device based on the file's layout metadata (or ​the format defaults if none are available) or downmix to 2.0 (preferably as a togglable downmixing mode option).