1 pass vbr: Allow for lookahead alt-ref in real-time mode.

1 pass vbr: Allow for lookahead alt-ref in real-time mode.

For 1 pass vbr real-time mode:
Allow for the usage of alt-ref frame when non-zero lag-in-frames is used.
Use non-filtered alt-ref, and select usage based on fast scene/content
analysis/detection within the lag of frames.

Positive gains on ytlive set: overall avgPSNR ~3-4%.
Several clips are up between 5-14%, a few clips are neutral/small change.

Current speed decrease is about ~5-10%.

Use the flag USE_ALTREF_FOR_ONE_PASS to enable this feature
(off by default for now).

Change-Id: I802d2bf3d44f9cf01f6d15c76be9c90192314769