Clean-ups on switchable interpolation and mv_ref

Adds backward adaptation and differential forward updates of switchable
interpolation filter probabilities. Also adds some cosmetic cleanups
and minor fixes on mv_ref probabilities.

derfraw300: +0.353% (with most coming from switchable interp changes)

Change-Id: Ie2718be73528c945fd0d80cfd63ca2d9cb3032de