Add basic erosion morphological operation

[coreh]

Hey there :wave:

While comparing images produced by two different computers via WebGL, we noticed that sometimes the images would include 1px-2px thick spurious diff regions, likely caused by differences in rasterization and floating point precision between GPU models.

pixelmatch already includes antialias detection which helps alleviate some of these false-positives, but in some cases they were still showing up in our image diffs.

This PR introduces logic to optionally apply an erosion morphological operation after calculating the diff — which eliminates 1-2px wide structures from the image diff — and to return the total of non-eroded pixels.

Erosion is implemented as a separate pass, so it requires the output image to be present. By default eroded pixels are marked with an orange color.