Unfortunately, this necessarily involves parsing codec frames to get
their durations. Ogg's granule positions indicate the last sample of
the last complete frame of the page, so the navigator has to determine
the durations of every packet in the page to offset it back to its
start.
Vorbis is an especially involved codec to determine that for, since it
has initial data defining a mapping of indices to big or small block
sizes that has to be held onto, and that mapping is preceded by a bunch
of conditionally parsed bits. Also, a frame's block size calculation
involves the block size of the previous frame for an overlap add. To
avoid bringing that complexity directly into LibMedia, we delegate the
parsing/calculation to FFmpeg's av_vorbis_parse functions.
Add a new FLACNavigator class that can scan for frames both forward and
backward from a byte offset to get the exact buffered ranges. The reads
are done in chunks, which keeps each call pretty cheap.
The ranges themselves are resolved by its ScanningContainerNavigator
base class, which reconciles cached ranges against the incoming ones to
avoid repeating work unnecessarily. With those combined optimizations,
the calls normally take under 1 microsecond.
