Add support for AVC-Intra

This format has been reverse engineered and x264's output has almost exactly
the same bitstream as Panasonic cameras and encoders produce. It therefore does
not comply with SMPTE RP2027 since Panasonic themselves do not comply with
their own specification. It has been tested in Avid, Premiere, Edius and
Quantel.

Parts of this patch were written by Jason Garrett-Glaser and some reverse
engineering was done by Joseph Artsimovich.