Add WebM support to the matroska writer.

Not really tested, since we're just stuffing AVC packets
into what claims to be VP8 data.
Add VP8 parameter
Initial VP8 header support
Need the RAC for the rest of it.
Add VP8 arithmetic coder (not at all tested)
More header stuff
Fix MKV issues, make ffmpeg open it again
More header stuff, probabilities are next
Split out vp8 header writing
Add basic probabilities and header stuff
All hacked in for now, no real contexts yet.
Finish ending the header
Fix encapsulation and stuff
Fix the RAC
Fix keyframe bit
Um, this seems to work?
Initial binarization code
Enable mb skip, to make it easier to produce a valid frame
We're just going to skip all blocks for now.
Set up the (RAC part of the) VP8 macroblock loop
Should in theory work at this point (but totally doesn't).
Set invisible bit correctly
Fix comment
"is frame referenced" bit isn't written in keyframes