…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/6493da33ecacf937e915726cb71f9f1ec3f68da1