VREyeParameters API incompatible with canted displays

[lonetech]

I have a Pimax headset with canted displays. The matrices produced by GetEyeToHeadTransform thus contain a rotation. Not only does openvr/display.rs discard that data in fetch_eye_parameters, there's nowhere to put it in VREyeParameters. This will need to be fixed, and maybe not just for wide field of view headsets; the Valve Index also has canted optics.

[jdm]

We would accept a pull request to fix this, but the plan is to replace this crate with https://github.com/servo/webxr/ in the next few months, so we don't expect to invest any time fixing bugs that don't directly affect hardware that we are running Servo on.