Add support to the yaml reader and writer to be able to specify
that a primitive should set the PREFER_COMPOSITOR_SURFACE flag.

This flag is not currently used, but in future will signal the
picture caching code to promote a primitive to draw on a native
compositor surface where possible.

Differential Revision: https://phabricator.services.mozilla.com/D62693

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/1e1ffcbadfb2350ecda25b8752b3c07c2f2d0722