Estimate angle histogram using gradient analysis, then skip those
angles that are unlikely to be chosen.

On ext-intra experiment, turning off filter-intra modes:
for all-key-frame setting, computation overhead is reduced
by about 40%, coding gain dropped from +2.08% to +1.96% (derflr);
with kf-max-dist=150, computation overhead is reduced
by about 60%, coding gain dropped from +0.58% to +0.49% (derflr).

Change-Id: I36687410fb10561b8e1a8eebb1528cf17755bd5b