Segment coding of mode and reference frame.

Proof of concept test code that encodes mode and reference
frame data at the segment level.

Decode-able bit stream but some issues not yet resolved.
As it this helps a little on a couple of clips but hurts on most as
the basis for segmentation is unsound.

To build and test, configure with
--enable-experimental --enable-segfeatures

Change-Id: I22a60774f69273523fb152db8c31f4b10b07c7f4