Support chained ogg files

[est31]

Apparently its legal to chain multiple logical streams inside an ogg/vorbis file. The vorbis spec explicitly allows it:

The Ogg/Vorbis I specification currently dictates that Ogg/Vorbis streams use Ogg transport streams in degenerate, unmultiplexed form only. That is:

    A meta-headerless Ogg file encapsulates the Vorbis I packets
    The Ogg stream may be chained, i.e., contain multiple, contigous logical streams (links).
    The Ogg stream must be unmultiplexed (only one stream, a Vorbis audio stream, per link) 

See also similar bugs filed for flac, in firefox, and opus.

We should at least support playback here.

[est31]

The use case is streaming apparently, when you want to update metadata while the stream is running.

[lu-zero]

In general the demuxer should issue what I call a new stream event and react to it by either allocating a new decoder if it is a new stream or resetting the current decoder pinned to the current stream.

If you have some test sample we could try to adapt your ogg demuxer to fit this model and make sure it works well enough.

[est31]

I don't have any test samples but I think you should be able to generate some by concatenating two distinct ogg/vorbis files (it is important that they are distinct so that the stream serials don't match).

The ogg crate and the low level API of lewton that rust-av uses are more or less ready for it. The ogg crate just spits out packets and the only state that the low level API holds must be destroyed one way or another. What does need a change is the glue code between the two. lewton provides its own glue code via the inside_ogg module. This glue code is not used by the av-vorbis crate however, more or less by construction (av-vorbis intends to be generic from the container, supporting e.g. matroska, while inside_ogg only concerns ogg). The purpose of the inside_ogg module is to allow "normal" users to parse ogg/vorbis files without much fuss which is 99% of what people want to do. So given that even if it is fixed in lewton, it won't automatically be fixed for rust-av, I think we should open a separate bug in the av-vorbis crate and track the issue there.

[est31]

There are test vectors in https://people.xiph.org/~xiphmont/test-vectors/vorbis/ (files chain-test1.ogg to chain-test3.ogg) . Commit e051c7e introduced initial support.

[est31]

The first and second chained file examples are now fixed with commits dbfaae5 and e2b38e8.