Compute global refmv candidate at center of current block
Compute global refmv candidate at center of current block

When a neighboring block uses global motion, use the mv
computed at the center of the current block as the candidate vector
rather than the mv computed at the center of the neighboring block.

0.15% improvement on cam_lowres

Change-Id: I79eff8bf27a7aa84ae4a6d56e4a10c41a4438fb9

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom