libavcodec apparently holds onto any error that is not AVERROR_EOF when
a read fails. This means that reading until EOF after an aborted read
results in us receiving an AVERROR_EXIT in FFmpegDemuxer instead of
AVERROR_EOF, which causes the playback system to enter an error state
without decoding all frames in the file.
Instead, just always return AVERROR_EOF, and check if the read was
aborted in FFmpegDemuxer instead to return the correct error category
from there.
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.
When a seek is requested while a previous seek is still blocked waiting
for not yet available bytes, we want to abandon the old request
immediately and start processing the new one.
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.
