Ensure that the image keys and image generations for external
compositor surfaces are included in the composite descriptor,
which is used to determine if a composite is required or can
be skipped.

Differential Revision: https://phabricator.services.mozilla.com/D65216

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/5f2a8eb4989755c3cf987738ea04034ac5bea896