These display some idea, but the tests are not implemented correctly. We
will need to implement the idea correctly later, but for now we can't use
these.
This change does not include the addition of VP9. We do that in a
separate pull request.
Changes:
* fix the video bug (video frames larger than 65KBytes) by sending full
frame length in alternate header field
* improve video frame reconstruction logic with slots
* configure video encoder and decoder to be multihtreaded
* set error resilience flags on video codec
* change encoder and decoder softdeadline