…r=nical

This makes the picture cache debug view more represent the amount
of pixels that are being rasterized and composited. It's also a
bit clearer where picture cache boundaries are on some pages.

Differential Revision: https://phabricator.services.mozilla.com/D65932

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/ca720bdc1f1fc3c41a278d10c3c14b9016bd76e1