How should timestamps be handled for audio discarded during decoding?

[dalecurtis]

While trying to fix the negative timestamp test failures in Chromium:

• https://wpt.fyi/results/webcodecs/audioDecoder-codec-specific.https.any.html%3Fopus?label=master&label=experimental&aligned&q=Test%20decoding%20a%20with%20a%20negative%20timestamp

I've run into a question about how things like Opus pre-skip should be handled. The opus test file in this case has a pre-skip of 312 samples (6500us), which Chromium's decoding system happily discards. To ensure timestamps are accurate we adjust the -42 forward (to 6548us).

As I see it, we have a couple choices:

• AudioDecoder shouldn't discard anything.
• AudioDecoder shouldn't adjust timestamps for discarded audio (not sure how this works when full packets can end up discarded).
• Agree that timestamps should be adjusted for discarded audio.
• Do nothing and switch to a Opus test file with zero pre-skip

@padenot @youennf @tguilbert-google WDYT?

[dalecurtis]

This is some what related to #626 and I could have sworn Paul and I chatted about this before, but I couldn't find anything in my e-mail.