The way that other classes interact with IncrementallyPopulatedStream
is now through a virtual interface MediaStream and MediaStreamCursor.
This way, we can have simpler implementations of reading media data
that will not require an RB tree and synchronization.
...a shared byte stream that can be appended to as network data arrives.
The stream supports creating multiple independent `Cursor`s, each with
its own read position. This matches our demuxing needs, where different
audio/video tracks may read from the same underlying data concurrently.
`Cursor::read_into()` and `Cursor::seek()` block until the requested
range is available (or the stream is closed). This is intentional: the
FFmpeg `avio_alloc_context()` read callback cannot reliably surface
`EAGAIN` without putting the demuxer into an error state that
requires recovery via seeking, so we instead wait until we can satisfy
the read.
This commit implements the functionality to play back audio through
PlaybackManager.
To decode the audio data, AudioDataProviders are created for each track
in the provided media data. These providers will fill their audio block
queue, then sit idle until their corresponding tracks are enabled.
In order to output the audio, one AudioMixingSink is created which
manages a PlaybackStream which requests audio blocks from multiple
AudioDataProviders and mixes them into one buffer with sample-perfect
precision.
