Allow for compound prediction mode in nonrd_pickmode for ZEROMV.
For real-time encoding, 1 pass with non-zero lag-in-frames.

Added speed feature to control the feature.
Enabled for speed >=6 for now, under VBR mode.

avgPSNR/SSIM metrics positive on ytlive set, for speed 6:
some clips up by ~3-5%, some clips neutral gain, average gain
across clips is ~1%.

Small/negligible decrease in speed.

Change-Id: I7a60c7596e69b9a928410c5ee2f9141eecd8613d