cdef: pick strength based on quantizer (#1401)

* cdef: pick strength based on quantizer

   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.3890 | -0.7035 | -1.2771 |  -0.1636 | -0.0987 | -0.0441 |    -0.1748

~4-6% faster encoding time for low QP.