Add a speed feature to seperate prediction mode and tx type search
for intra modes: search for best intra prediction mode with fixed
default tx type first, then choose the best tx type for the
selected mode.

Coding performance drop:
baseline
  lowres 0.10% midres 0.08% hdres 0.14%
with ext-tx
  lowres 0.14% midres 0.25% hdres 0.20%

Speed improvement is 20% for baseline and 17% for ext-tx.

It is turned on for speed >= 1.

Change-Id: Ia5e8d39e8a4e2e42c521bfde938f8b6a98ab24f9