vp9: Allow usage of lookahead for real-time, 1 pass vbr.

vp9: Allow usage of lookahead for real-time, 1 pass vbr.

Allow usage of lookahead for VBR in real-time mode, for 1 pass vbr.

Current usage is for fast checking of future scene cuts/changes,
and adjusting rate control (gf interval and active_worst/target size).

Added unittests (datarate) for 1 pass vbr mode, with non-zero lag.

Added an experimental option to limit QP based on lookahead.

Overall positive gain in metrics on ytlive set:
avgPNSR/SSIM up on average ~1-3%; several clips up by 5, 7%.

Change-Id: I960d57dfc89de121c4824b9a9bf88d2814e74b56