Ability to set strict z-ordering of CSS VR content.

[caseyyee]

Currently, all CSS VR content share the same 3D context. This can be a problem in cases where we want elements to always render on-top regardless of their spatial positioning (ie. for HUD elements).

One solution could be to use the css z-index property.

[vvuk]

The solution for this is proper depth buffering, which isn't available yet. It's on kip's list of things to look at once he finishes up with the transform bugs.

[caseyyee]

FIled into bugzilla https://bugzilla.mozilla.org/show_bug.cgi?id=1171771