The layers.use-image-offscreen-surfaces preference was only really useful during a transitional period where we wanted to keep xrender for compositing but disable it for content rendering. Since we have moved to universally disabling xrender, this preference just becomes an annoyance when a user wants to toggle xrender back on in, for example, remote usecases.

This patch makes xrender all-or-nothing based on the gfx.xrender.enabled preference. You either get all of xrender or none of it, which makes it easier to figure out which prefs you need to toggle.

May shelve this until we determine if we just want to always force use-image-off-screen-surfaces instead, as that would let us not have to deal with xrender for content in any case. The issue is I guess if we can still get around our performance issues in remote X setups by allowing compositing to use xrender even if not content.