Output trimming for future BYOB

[chcunningham]

BYOB is blocked on having read-only buffers, tracked in WICG/reducing-memory-copies#1. This issue adds a consideration for when that is eventually resolved.

The request from audio industry pros is for an ability to say "when you write your output to my buffer, please skip over X bytes and stop writing after Y bytes". The use case is for codecs that are known to produce throw some initial throw-away outputs, like opus and mp3.

[padenot]

and AAC, and Vorbis, for completeness about formats commonly used on the web.

This is similar in spirit to what MSE provides, https://w3c.github.io/media-source/#sourcebuffer-audio-splice-rendering-algorithm, except here it's for saving some CPU and memory traffic, not to implement gapless audio.

[chcunningham]

Triage note: marking 'extension', as doing this for BYOB would involve new arguments to decode() or configure().

Separately, I want to highlight that we are now offering this feature without BYOB as part of the AudioData redesign in #228