This commit allows encoder to skip intra coding mode test, when
the known inter residual is less than the source variance. It
reduces the runtime of speed 3 for test clips:
bus cif 1000 kbps: 8587 ms -> 8260 ms, 3.8% speed-up
pedestrian 1080p 2000 kbps: 161381 ms -> 155241 ms, 3.7% speed-up.

The compression performance is down by
derf   -0.36%
stdhd  -0.25%

Change-Id: I75ce1e035b4da2153cb1ac14111d1a07c05a735d