d/ohmd: Add position tracking.

This has Monado use the OpenHMD device's tracked position if the device's OpenHMD driver provides it.

I know there aren't many official OpenHMD drivers that actually provide position tracking. I've been using a fork of OpenHMD based on https://github.com/ChristophHaag/OpenHMD/tree/libsurvive which uses libsurvive to achieve position tracking with the HTC Vive.

Ideally, Monado's OpenHMD wrapper driver would also set the orientation/position VALID and TRACKED space relation flags based on the OpenHMD device's rotational/positional TRACKING device flags, but this seems to be okay for now since the OpenHMD drivers that don't support position tracking tend to set the position vector to zero anyway.