libhb: Discrete channel settings for nlmeans.
libhb: Fix math bug in nlmeans border copy.
libhb: Skip nlmeans for channel if strength == 0.
libhb: Make some nlmeans variables const.
libhb: Cosmetic improvements to nlmeans.
libhb: Fix math bug in nlmeans border copy (again).
libhb: Fix nlmeans bug causing zeroed out pixels.