Memoize faded colors in overlayed comparison images

Since many web pages contain large blocks of the same color (many pixels
are identical), we can save a significant amount of time in composing
the faded colors on opaque white colors by memoizing. In my
non-scientific tests, this seemed to make the image comparison about 40%
faster. We might be able to see even better speed improvements by
pushing the memoization a step up the chain so it happens before the
fade step.

Addresses #76.

Change-Id: I64d11a94d8e20945c1f651f9cc6e08cb5a0cafaa