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In RFC 7741, PictureID is not mandatory and
required only if SLI is used.

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This was added in draft-ietf-payload-vp9-10.

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Relates to pion#87

A MTU can never be negative so this removes a class of runtime errors.

Chrome need SPS and PPS in one STAP-A, and
timestamp of STAP-A should be same with next
I-Frame. Otherwise Chrome will discard SPS and PPS
before receiving I frame.

We used to do partition head checking in PartitionHeadChecker and
partition tail checking in Depacketizer, which is confusing and
error-prone.  Move all checking into the Depacketizer itself.

WebRTC clients use padding only packets as probe
packets for bandwidth estimation. As padding was
not removed from the payload, downstream code
trying to parse padding as valid media payload
results in erroneous parsing

Testing:
--------
- Add unit tests for different padding scenarios
- Check that ion-sfu does not try to parse padding only
packets as valid video payload.

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When a packet has padding, the padding size information
is lost during unmarshal. Add a PaddingSize field to
Packet structure.

Also support marshalling on an RTP packet with padding.

Testing:
--------
Modified/added unit tests.

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MTU was changed to an unsigned integer

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This change adds an interface and impl for zero-copy extensions.

This change makes the H265 packet satisfy the depacketizer
interface allowing H265 streams to be decoded with
SampleBuilder.

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Marshal, MarshalTo, MarshalSize, Clone functions shouldn't have a
pointer, since they're constant. This convention is used by pion/rtcp
and the standard library, and prevents coding errors.

Implements Marshal and Unmarshal support. Marshal doesn't pack multiple
OBUs into one packet and could be improved.

Missed when cherry-picking from master

Codecs only contains RTP Payload processing

Only take from OBUs slice if greater then 0

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Allows packetizer marshal / unmarshal to be consistent

Resolves pion#79

Add AbsCaptureTimeExtension and PlayoutDelayExtension implementations.
Both of these are experimental RTP header extensions defined in
libwebrtc.

Multiple optional fields of the VP9 RTP header are read regardless of
the buffer size. This can easily lead to multiple crashes. This PR add
the necessary checks in order to avoid crashes.

All of the fields in the VP8 header except the first byte are
optional.  We used to reject VP8 packets smaller than 4 bytes,
which is incorrect.

There are two cases where such packets may appear on the wire.
GStreamer's WebRTC implementation generates VP8 streams with
no picture id, and one-byte headers.  It will occasionally
generate packets that are below 4 bytes, and which we used
to reject.

The second use case is more theoretical.  According to RFC 7741
Section 4.4, a packetizer may ignore VP8 partition boundaries.
If it splits a packet outside of a partition boundary, it may
generate a packet with S=0 and a one-byte header.

This prevents gofmt -w -s from messing up existing code.

Text with multiple spaces is now wrapped in long comments (/* */), that
are not affected by gofmt.

Remove checks since payload size can be less than 2.

Only FU-A explicitly accesses the second byte, which is not affected
because a separate boundary value check is performed.

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Clone performs deep copy on a packet to produce equivalent
but independently mutable packet.

Fixes pion#88