Support for edit lists

[cconcolato]

The BSF spec indicates that implementations must support one type of edit list (called "offset edit list" in CMAF) but is silent about other types. Amongst other types we have:

1. use of fractional rate,
2. use of zero rate (i.e. dwell),
3. use of rate = 2.
4. use of 'empty' edits (media time -1)
5. use of multiple entries

In my testing, there does not seem to be interoperability. Most of the time browsers ignore the edit lists they don't support. It'd probably be better to fire an error.

[wolenetz]

From Sep 28 Media WG teleconference (https://www.w3.org/2021/09/28-mediawg-minutes.html), we had agreement to not regress existing content playback behavior, but to instead clarify expectations of implementations and provide guidance to content providers:

1. Document that implementations MAY ignore these other kinds of edit list situations.
2. Document that content providers SHOULD NOT rely on these other kinds of edit list situations.
3. Add a non-normative note that use cases that are not solved by the MSE API in combination with the required support for the "offset edit list" can best be discussed by filing an issue against the BSF spec.
4. Separately, implementations can internally do some data gathering to find how often these other kinds of edit lists are used with MSE (even though they MAY be ignored), to assist in identifying potential use case gaps or interop issues.