Retune the CLPF kernel

CLPF performance had degraded by about 0.5% over the past six months,
which isn't totally surprising since the codec is a moving target.
About half of that degradation comes from the improved 7 bit filter
coefficients.  Therefore, CLPF needs to be retuned for the current
codec.

This patch makes two (normative) changes to the CLPF kernel:

* The clipping function was changed from clamp(x, -s, s) to
      sign(x) * max(0, abs(x) - max(0, abs(x) - s +
             (abs(x) >> (bitdepth - 3 - log2(s)))))
  This adds a rampdown to 0 at -32 and 32 (for 8 bit, -128 & 128
  for 10 bit, etc), so large differences are ignored.

* 8 taps instead of 6 taps:
               1
    4          3
  13 31  ->  13 31
    4          3
               1

AWCY results: low delay  high delay
PSNR:           -0.40%     -0.47%
PSNR HVS:        0.00%     -0.11%
SSIM:           -0.31%     -0.39%
CIEDE 2000:     -0.22%     -0.31%
APSNR:          -0.40%     -0.48%
MS SSIM:         0.01%     -0.12%

About 3/4 of the gains come from the new clipping function.

Change-Id: Idad9dc4004e71a9c7ec81ba62ebd12fb76fb044a