libvpx

Shorten GF/arf interval in hard scenes.

This patch accounts in the first pass stats for blocks that
while not coded as intra, are complex and have an intra error /
best error ratio below a threshold.

The modification shortens the GF arf interval for a particular
class of content that contains a lot of blocks matching the
above criteria. (In one short problem test sequence the average
interval dropped from about 14-15 to 10-11)

The change results in small net gains in metrics results for the
Yt(~0.2%) and yt-hd (~0.5%) sets and is approximately neutral
for the other test sets.

The change is currently shielded by a flag and off by default
pending verification that it does not cause other regressions
in tests on a wider YT test set.

Change-Id: I6b803daa6a4ac09a6f428fb3a18be1ecedd974b7