Fix bug with source audio

If the audio didn't start at the 0 timestamp, it would misinterpret it
as a timestamp jump because obs_source::next_audio_ts_min is set to 0 on
creation.  Timestamp starting values should be allowed to start at any
arbitrary value.
win-dshow: Add audio support

This implements audio support, allowing not only the ability to capture
the built-in audio from the video device's audio capture pin, but also
the ability to override the default audio with a custom audio device.

The DShowInput::Update function was split up and refactored a bit, as it
was getting a bit large and messy.