[codec] Ogg Opus zero-length frames

[Mark Harris <[e-mail]>]

According to draft-ietf-codec-oggopus-01 section 4, Opus frames
with length 0 can be used to fill in gaps when writing Opus to Ogg,
since no gaps are allowed.  That makes sense, and is useful for
representing frames that were lost, corrupted, or not transmitted
due to DTX.  However those frames need to go in packets, which
require a TOC.  That means that in order to represent packets that
are not available, the muxer must synthesize a TOC.

It would be nice to have guidelines for encoding these gaps in the
Ogg Opus draft, including:

  * Clarification that zero-length frames should be written, with
    a synthesized TOC if necessary, in the case of missing frames
    (e.g. due to packet loss or corruption).  Currently section 4
    only mentions these frames in the context of an encoder with
    DTX enabled.

  * Clarification as to whether the LP/Hybrid/MDCT mode and stereo
    bit matter for zero-length frames, and if so then a recommended
    method of choosing these when it is necessary to synthesize a
    TOC.  This may be influenced by the duration of the gap, since
    not all valid gap durations are possible to encode in LP mode.

  * Clarification as to whether the number and duration of the
    individual frames matter, as long as the total duration is
    correct, or a recommended method of choosing frame durations.
    For example it takes fewer bytes to encode a 95ms gap as 19 5ms
    frames (using a single code 3 CBR packet), but if the previous
    packet was LP mode it may be better to use multiple packets
    with longer frames in order to remain in LP mode as long as
    possible.  There may also be a benefit to keeping the previous
    frame size as long as possible.

  * Clarification as to whether zero-length frames may be used in
    code 0 or code 1 packets, since RFC 6716 section 3.2.1 is
    referenced which only mentions their use in code 2 and code 3
    packets.

  * Clarification as to whether it is acceptable to combine
    synthesized zero-length frames with other longer frames in the
    same code 2 or code 3 packet, when the TOC matches.

  * Recommended method of recording gaps that are not a multiple of
    2.5ms.

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