Adds a new interpolation experiment.

Improves entropy coding to send the filter type only if
the motion vectors have subpel components.
Adds one new 8-tap smooth filter, and tweaks the others.

derflr: +0.695%
hevcmr: +0.305%

About 5% encode slowdown. No visible impact for decoding.

Also makes the interpolation framework flexible to support both
strictly interpolating filters as well as non-interpolating
filters that filter integer offsets. This is mainly for
further experimentation and if not found useful the code will
be removed.

Change-Id: I8db9cde56ca916be771fe54a130d608bf10786e6