Optimize wedge partition selection.

We can optimize wedge partition selection by pre-computing the
residuals of the 2 underlying predictors, and then blend these
to compute the sse of the compound predictor, without actually
having to compute and subtract the compound predictor.

Similarly we can pre-compute a proxy array which we can use to
cheaply check which mask sign would have lower sse.

Details are in wedge_utils.c.

Mathematically these are equivalence transformations, but due to the
finite precision the encoder output will be perturbed, though on
average this should make 0% difference.

ext-inter gains about ~4.5% speedup.

Change-Id: Ib2657c3209ae161b4090b58b4b6c392641bf2792