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.
...and abstract away the stream/cursor blocking/aborting functionality
so that demuxers can implement or ignore those methods as they see fit.
This is a step towards implementing a wrapper demuxer for MSE streams.
It's not necessary to keep around an instance of AVFormatContext in
FFmpegDemuxer, so instead just copy out the info we need for our
implementation and then destroy it so that our stream cursor is freed.
Refactor the FFmpeg and Matroska demuxers to consume data through
`IncrementallyPopulatedStream::Cursor` instead of a pointer to fully
buffered.
This change establishes a new rule: each track must be initialized with
its own cursor. Data providers now explicitly create a per-track context
via `Demuxer::create_context_for_track(track, cursor)`, and own pointer
to that cursor. In the upcoming changes, holding the cursor in the
provider would allow to signal "cancel blocking reads" so an
in-flight seek can fail immediately when a newer seek request arrives.
By default, MatroskaDemuxer chooses not to seek if the current frame
is closer to the seek target than the keyframe that precedes the seek
target. However, it can be desirable to seek to a keyframe anyway, so
let's allow that.
Most users will only care about the total file duration, and shouldn't
be required to determine the file duration from multiple track
durations. To facilitate that, add a total_duration() function that
returns the demuxer's duration not associated to any particular track.
The existing conversion was rounding to the nearest millisecond, which
is much less precision than most videos will want. Instead, use only
integer math to directly convert the presentation time to seconds and
nanoseconds for our AK::Duration to represent accurately.
