Add percentage diff to the image comparison

[bahmutov]

And do not update the images that changed in just a few pixels

[bahmutov]

Otherwise, we see commits like this from CI

Updating 38d1cc6..d34a1e6
Fast-forward
 images/demo-test.png | Bin 144115 -> 144109 bytes
 1 file changed, 0 insertions(+), 0 deletions(-)

Probably can use 0.1% byte size at first?