…es r=jgilbert

Turn the difference checkbox into a radio that adds "heatmap"; it uses
WebGL to show both images, their absolute difference, and a color-coded
max difference.  The quadrants split following the mouse.
This helps to separate large variations (red) from small variations
(green) and helps to compare the images without losing track of where
they are.

Differential Revision: https://phabricator.services.mozilla.com/D65841

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/263ce25c220aeab11843445e3634f29b004f44dc