…. r=gw

In bug 1574493, we moved most snapping to scene building and a minority
to frame building. No snapping is done in the shader. However there was
some left over code that still tried to replicate the past behaviour and
this caused wobbling during the rendering. This patch removes the extra
snapping on the CPU side and trusts scene/frame building to do the job.

Differential Revision: https://phabricator.services.mozilla.com/D61590

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/9867a7bc911e2d3abc2312826c9f14cfa7b10a63