Segmentation Entropy and tweaks.

Some correction for entropy impact of segment signaled (EOB and ref frame)

Other slight tweaks.

Derf VBR average gain now over 1% (best over 7%)
One YT test clip has gains of circa 30% (VBR)

There is still an issue with noisy clips where making the background static
and coded with 0,0 can have a negative effect, especially at low Q.
This is probably because of the loss of smoothing by fractional pixel filters.

Change-Id: I7a225613c98067b96f8fc7a7e36f95d465b2b834