Previously, the prim origin needed to be stored in the prim
instance, to avoid picture cache invalidations. With support
for external scroll offset, this is no longer necessary.

This simplifies some of the code paths, and reduces the size
of primitive instances.

Differential Revision: https://phabricator.services.mozilla.com/D70740

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/1f65f78fb36707cd936882f4d363f7c5f8a3314d