reftest analyzer's outline of differing area is confusing when the differing area touches the edge of the image

[dbaron]

The reftest analyzer is designed to put a red outline around the areas of the image that differ. However, it doesn't really work when the differing area touches the edge of the image, since the outline isn't drawn outside the image. This can lead to a confusing visualization when the image difference includes large swaths of the image's edge, such as in this example (from crrev.com/c/3638193).

Ideally the red outline would continue around 1px outside the edge of the image when that's necessary.