avfilter/dctdnoiz: rewrite [f/i]dct

This removes the avcodec dependency and make the code almost twice as
fast. More to come.

The DCT factorization is based on "Fast and numerically stable
algorithms for discrete cosine transforms" from Gerlind Plonkaa &
Manfred Tasche (DOI: 10.1016/j.laa.2004.07.015).