add test with completely different images
for each dimension, sample every 8th pixel

provides a good balance between accuracy and performance.

on a representative retina screenshot diff computes Delta in 0.34s versus 0.79s with 99% accuracy (1.12% diff versus actual 1.13% diff)

2.4GHz Core i7
bugfix: accurately compute pct diff taking stride into consideration

TODO consider using a closed form solution to compute num pixels sampled given stride
test accuracy on an image that is 50% different

to take regressions