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