Improve codecs (related to #306)

[ibc]

• VP8: Check that the payload has pictureId and tl0PictureIndex before using them.
• H264: Check tl0picidx is greater than highest seen (related to Improve H264 temporal layer switching #306).

However, note that while in VP8::PayloadDescriptorHandler::Encode() we check whether pictureId is greater than highest seen, we don't have "picture Id" in H264, but just the tl0picidx. So I've implemented the same logic but by checking tl0picidx. I hope it's also valid.

[ibc]

NOTE: It strongly worries me the fact that we have a currentTemporalLayer in both SimulcastConsumer and EncodingContext classes, and they are not in sync but independently managed.

[jmillan]

NOTE: It strongly worries me the fact that we have a currentTemporalLayer in both SimulcastConsumer and EncodingContext classes, and they are not in sync but independently managed.

Agreed, I think we could use the EncodingContext to set, get the current temporal layer from SimulcastConsumer

[ibc]

However, note that while in VP8::PayloadDescriptorHandler::Encode() we check whether pictureId is greater than highest seen, we don't have "picture Id" in H264, but just the tl0picidx. So I've implemented the same logic but by checking tl0picidx. I hope it's also valid.

And it was not valid so has been reverted. This was letting MOST of the packets pass. This is because many H264 packets with different tid have the same tl0picidx. So check removed.