This patch adds a grid border around the image so that the SSIM
resolution doesn't drop for the border pixels.
We also add a test with WebKit rendering artifacts to make sure
new approach helps to fight this.
While migrating Playwright tests from pixelmatch comparator to the
ssim-cie94 comparator, we noticed a rendering artifact that happens
on playwright screenshot tests on WebKit Linux.
This PR adds a set of cropped-down examples and tunes the ssim-cie94
comparator to accept these rendering artifacts.