Modify ransac to keep the N best global motions by num_inliers and
variance rather than a single one. Compute the error advantage for
each in encode_frame_internal(), and use the best as the global motion
for that pair of <target, reference> frames.

Improvements for different values of N:

N                     %PSNR gain on lowres
1 (current impl)      1.287
2                     1.328
4                     1.370
8                     1.419
16                    1.427
32                    1.439

Change-Id: Ic0c9066a3f175a5ea0a78828cd244104e70144ba