…l tree r=gw

This change rewords get_relative_transform and assotiated pieces of logic,
so that we flatten the transforms at preserve-3d context boundaries.

It addresses a problem found by 1524797 but doesn't resolve the bug yet (!).
There is another issue likely contributing here, and we can treat this PR
as WIP and not merge until the case is completely resolved.

Differential Revision: https://phabricator.services.mozilla.com/D19254

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/87dac4d84c6b78511849fe117c327620980b4ee2