Allow specifying a custom diff renderer

[ansman]

We render diffs with a bit more detail than what Paparazzi does. Today this means we have to pass a completely custom handler which is a shame. It would be great if you could provide a way to just render diffs.

[jrodbx]

We render diffs with a bit more detail than what Paparazzi does.

Can you elaborate on what extra detail looks like? Might be useful to upstream some of that.

[ansman]

I wrote a 2x2 grid of images. On the top row is the expected and actual.
On the bottom row are two diffs. One shows a grayscale diff where white means identical and black is full difference (as defined by our pixel differ). The other diff was a highlighted diff where any differing pixels were magenta for visibility.

The idea of was that the grayscale diff is good for determining if the diff is noticeable and the magenta was good for finding the diff when it was not noticeable