libvpx

Improve motion detection for low complexity regions.

Where there is very subtle motion, especially when combined
with low spatial complexity, the codec sometimes fails to quickly
pick up the ambient motion field.

Once it has been established though the field propagates well using
Nearest and Near MV.

This patch looks specifically at the case where the Nearest and Near
have not been established as non zero vectors and in this case
discounts the cost of searching for a new vector in the rd code.

This will almost certainly have some implications in terms of encode
speed but it should be possible to mitigate the impact in a subsequent
using first pass stats and the local spatial complexity.

Average results for test sets approximately neutral.

Change-Id: I44a29e20f11f7ab10f8c93ffbdc50183d9801524