Only render overlay magenta on pixels that changed

I'm getting rid of the magenta "shimmer" that was rendered for every row
that was changed in the overlay diff image. Looking at the diff without
the shimmer made it much easier to focus on the exact place where the
change is.
Base the diff percentage score on `sdiff.size`

We were calculating the percentage diff score for a snapshot based on
number of changed lines divided by the max height of the before and
after image. This had the potential of making the diff larger than 100%
since the number of changed lines can actually exceed the height of any
of the images. This would be the case if there was a lot of added and
deleted lines. We are now instead using the total count of lines that
`Diff::LCS.sdiff` reports.

This had the added benefit of removing the need for the max_height
altogether.