For each superblock, keep track of how far from current frame
was the last significant content change, and use that (along
with GF distance), to turnoff GF search in non-rd pickmode.

Only enabled for speed >= 8.

avgPNSR on RTC/RTC_derf down by ~0.9/1.2.
Speedup on mac: ~3-5%.
Speedup on arm: 3.6% for VGA and 4.4% for HD.

Change-Id: Ic3f3d6a2af650aca6ba0064d2b1db8d48c035ac7