Random Access Point clarifications

[robUx4]

Following the discussions from #419, we need some explanations on RAP.

Historically the keyframe flag was not always set for audio blocks even though each audio (Simple)Block could be seeked to. So there's some room in the rules for that.

In the thread we also proposed a new flag (in the Simple|Block or in the Track) to specify how strict it is to the rule. With such a flag we could make rules stricter for future files as long as they use that flag.

Another rule that may not correctly apply to existing files is the fact the ReferenceBlock must list all the frames it references. I'm not sure this is the case. We should study what mkvmerge/libavformat/libwebm do.

This patches sits on top of #495 so it's a draft for now.

[robUx4]

• libavformat is only writing one ReferenceBlock and it does not always use the proper value:

    if (blockid == MATROSKA_ID_BLOCK && !keyframe)
        put_ebml_sint(pb, MATROSKA_ID_BLOCKREFERENCE, track->last_timestamp - ts);
    track->last_timestamp = ts;

The referenced frame may not always the previous one. And for a B-frame there are 2 frames to reference.

• libwebm also only writes a single reference, but that is normal since VP8 and VP9 don't have B-frames. It also gets the actual reference frame timestamp, but it seems to write as an absolute value instead of a relative value.

• mkvmerge seems to reference the proper frames and it seems to store them as relative timestamps.

[robUx4]

This is actually a duplicate of #555 which more comments and more up to date text...