Added an audio format for Opus, which also required a resampler to make sure that all imported sounds are converted to a compatible sample rate.
Added the speex resampler from the opus tools package to the third party source, including built libraries for windows and mac.
Changed FVorbisAudioInfo so that it inherits from an interface for any kind of compressed audio, which can be used everywhere instead of being wrapped in #WITH_VORBIS.
Added FOpusAudioInfo to decompress Opus data, not sure at this point whether it's only going to be used for streaming audio but works for non-streamed playback.
[CL 2056352 by Matthew Griffin in Main branch]
