libhb: Add median prefilter to nlmeans.



git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6223 b64f7644-9d1e-0410-96f1-a4d463321fa5
Branch "nlmeans" committed:

https://trac.handbrake.fr/changeset/6223
HandBrake/HandBrake@94e297ae0e76058b60642a73b37dcfd6601c065d