Improve rd_variance_adjustment() for low variance blocks.

Improve rd_variance_adjustment() for low variance blocks.

Change the cross over point for switching between per pixel
and per block variance numbers when comparing reconstruction
and source complexity.

This improves the accuracy of the comparison for low variance
regions, For example, recon and source may both have an integer per
pixel variance of 1, but one of these may actually be be 1.01 and the
other 1.99.

The reason for using per pixel at all was because this number is already
available for the source block so does not need to be recomputed
here. Changing the threshold from >0 to >100 for using per pixel values
will thus cause a little extra work for some blocks.

With my default runs on derf and nf sets their is a net gain as follows:
(-ve = better, Overal PSNR, SSIm, PSNR-HVS)

derf low res  -0.106, -0.107, -0.093
midres -0.000, -0.021, 0.001
hd res -0.198, -0.190, -0.282
nf2k -0.090, -0.088, -0.077

Change-Id: I53ef514fe1c35ee3f08c64e9b22fc05fc7fe5887