Optionaly filter 8x8 skip blocks, but still leave superblocks that are
completely skipped unchanged.  Use one bit in the dering level to
signal replacing the signal for dering damping.

This works better with cb4x4 that now is enabled by default.

Low latency, used-cpu=4 change:
   PSNR|PSNR Cb|PSNR Cr|PSNR HVS|   SSIM|MS SSIM|CIEDE 2000
-0.6260|-0.3263|-0.3797| -0.3574|-0.7072|-0.4828|   -0.6584

Change-Id: I42c4290b52a50779770884fbabb020fdb1034ff7