We only need to get the frames from a block when requested by the
demuxer, so factor that out into a function that it can call when it is
outputting frames.
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.
This is important for seeking Opus tracks, as Opus needs to decode a
certain amount of input data before its output converges. Without this,
audio after a seek can sound muffled briefly.
We don't need to put these in a union, it's not gonna save us much
space. The reader also may find both audio and video elements, in which
case this code would produce undefined behavior.
Also, with this change, we can use the default values from the spec.
This first pass only applies to the following two cases:
- Public functions returning a view type into an object they own
- Public ctors storing a view type
This catches a grand total of one (1) issue, which is fixed in
the previous commit.
