On windows, when a source has only audio (no video) yet is marked as
capable of both video and audio, it would be programmed to expect a
video frame to synchronize with. This fixes that potential issue by
detecting whether any video is actually playing or not.
Adds functions to turn on audio monitoring to allow the user to hear
playback of an audio source over the user's speaker. It can be set to
turn off monitoring and only output to stream, or it can be set to
output only to monitoring, or it can be set to both.
On windows, audio monitoring uses WASAPI. Windows also is capable of
syncing the audio to the video according to when the video frame itself
was played.
On mac, it uses AudioQueue.
On linux, it's not currently implemented and won't do anything (to be
implemented).
