Allow tx search for ARF and GF helps quality but a little slower.
Setting subpel_iters_per_step to 1 improves encode speed.
Setting sf->mode_skip_start = 10 improves speed.

Initial local results suggest overall impact on quality is neutral
but encode is up to 15% faster.

Change-Id: Ibde02cae6626a44c10a1da0cefe888afbb51f037