Unit Test Image Comparision Improvements

This PR improves the image comparison we use in the unit tests to be a little more forgiving.  We now do the following steps to compare:

 - Pick the smaller of the images... either the frame image or the reference image.
 - Resize the images down to the common size using nearest neighbor filtering.
 - Convert the image to greyscale to mask color differences not perceptible to the eye.
 - Compute the absolute delta between the two greyscale images.
 - Count the number of pixels above a tolerance of 3.

This is a pretty standard method of comparing images for similarity.  Most of all the visual unit tests now pass as they should.

There are 3 visual unit tests which fail, but there seem to be bugs in the rendering.