Changes to make VP9 encoding work

As with audio, you have to be willing to keep sending more frames into
the encoder until it's ready to cough up a compressed frame.

Currently libvpx is only working well with variable bitrate and the
--cpu-used=2 option.
More VP9 related fixes

Got to make sure the video codec isn't asked to spit out another frame
right away after it finalizes. That's just for the VBR phase.
Merge branch 'vp9_fixes'