…g invalidation when zoomed in. r=aosmond,botond

* Fix crash due to shift left causing overflow (debug only)
* Remove rounding of scrolling offsets and snap to view space instead of
world space

Differential Revision: https://phabricator.services.mozilla.com/D57017

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/03fe3d76bd48bdd9be40568a136c131b1d129460