libhb: Add edge boost prefilter to nlmeans.

Simple filtering methods such as mean 3x3 or mean 5x5 inherently attenuate edge detail. Edge boost uses a variation on the Sobel kernel to find and partially reconstruct edges by blending pixels from the original image with the prefiltered image.
libhb: Reserve and reallocate some prefilter mode enum values in nlmeans.
libhb: Split nlmeans_border into two functions (nlmeans_alloc).
libhb: Recreate borders following prefilter in nlmeans.
libhb: Rename some nlmeans variables, e.g. w_cropped -> iw (inner width).