Avoids including unnecessary headers within the audio_renderer.h header,
lessening the likelihood of needing to rebuild source files including
this header if they ever change.
Given std::vector allows forward declaring contained types, we can move
VoiceState to the cpp file and hide the implementation entirely.
GetAudioRendererSampleRate is set as a "STUB" as a game could check if the sample rate it sent and the sample rate it wants don't match. Just a thought of something which could happen so keeping it as stub for the mean time
