The AV1 packetizer did not properly packetize all AV1 bitstreams. It
incorrectly calculated the "w" field from the AV1 RTP spec once and
applied it to all RTP packets. This field needs to be determined on a
per-packet basis. In practice, this meant the packetizer generated
invalid RTP packets if w = 0 for the first packet (i.e. 4 or more OBUs).

Rewrite the packetizer to accurately determine how many OBU fragments
are present in each RTP packet. The general idea is:
1. Start with the first OBU in the user buffer
2. Fill the RTP packet with data from the current OBU
3. If there is space left over in the packet, move to the next OBU and
   repeat #2
4. Once the RTP packet is full, report it to the handler and clear it
   and repeat #2

Update the tests to get wider coverage of this new algorithm. Add
another stream with more OBUs and test each stream with varying packet
sizes. Also, fix the frame re-assembly code in the test to conform to
the spec.

Name this new algorithm av1_packetize_new and keep the old
av1_packetize_high around for now. Library users can switch to the new
implementation for testing until it replaces av1_packetize_high.

AV1 RTP spec: https://aomediacodec.github.io/av1-rtp-spec/

Co-authored-by: Nicholas Cook <nicholasc@nvidia.com>