Add VR settings UI

[fredemmott]

This includes position, rotation etc. The settings already exist in the SHM, and are respected by the renderers, but there is no way to change them.

There are two Y values:

• eyeY: distance below the nominal eye height. This is primarily used by games that use the Oculus SDK, and introduced VR support back when it was intended as a seated experience. 'Floor level' was not a concept in the SDK originally.
• floorY: distance above the floor. Used by games with an awareness of 'roomscale'.

'Eye level origin' is arguably a better choice for cockpit-based games, but for Oculus games, we have to respect the games' decision, whichever way it is.

SteamVR (and OpenXR) let us choose on a per-layer basis, so we don't need to stick with what the game chose. Distance from floor seems to feel better for OpenKneeboard: set it to your floor-to-top-of-knee height, and it'll be in the right place.

That said, it might be worth adding a preference setting (and SHM flag): it's plausible that 'eye level origin' works better for people with some cockpit setups.