The deringing filter documentation is in:
J.-M. Valin, The Daala Directional Deringing Filter, arXiv:1602.05975 [cs.MM],
2016.
http://arxiv.org/pdf/1602.05975.pdf

ntt-short1 results:
          MEDIUM (%) HIGH (%)
    PSNR  -2.488088 -2.003235
 PSNRHVS  -1.588932 -0.797850
    SSIM  -1.522767 -1.287861
FASTSSIM   4.307822  3.983496

subset1 improvement is around 2-3% on PSNR (but the rate is mostly outside
the AWCY testing range)

Change-Id: Ic02344ce9faa509f5c3a50a0fb7a7b84a7977e72