…r=gw

we expect that the children spatial node goes after the parent.
If it's not the case, we can still fall back to a full world transform,
but it's not going to be correct with regards to flattening.
As we don't know the circumstances and are unable to reproduce the issue,
making this fallback could be a reasonable thing to do for now.

Differential Revision: https://phabricator.services.mozilla.com/D69289

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/085e6b728abbe83e92022f6aeeb157f240898fcf