Increase untouched buffer timeout when some of the buffers have been
touched. Might improve audio quality on games that suffered from
miniscule popping even when buffering was enabled (such as DeS).
In addition, made time stretching algorithm slightly more aggressive.
Includes some other tiny tweaks as well.
Also renames "AudioThread" to "AudioBackend". The new name is more
descriptive of what the class really is responsible for, since the
backends are not responsible for managing the audio thread.
NOTE: Right now only XAudio2 is supported