Implement XR_HTCX_vive_tracker_interaction for openxr

[TheButlah]

SteamVR supports XR_HTCX_vive_tracker_interaction, which is how openxr applications can get FBT on openxr. Do do so, steamvr uses the openvr driver (like the one in this repo) and exposes it via this openxr extension.

However, IIRC this is a steamvr-specific feature, and its not clear to me that doing the same with monado is even possible. This is because runtimes like monado are only OpenXR and do not use the OpenVR api. Instead, they must use software like xrizer or OpenComposite to emulate OpenVR on top of OpenXR.

It would be good if SlimeVR had native openxr support, to facilitate runtimes other than SteamVR (such as monado) or for those users that want a pure OpenXR setup and not rely on api emulation. I'm not knowledgeable enough about openxr to say the right way to do it (OpenXR layer? Whats the normal way to do this type of thing?). Perhaps @kitlith knows, or knows who to ask.

[kitlith]

Grain of salt, I have been completely uninvolved with the efforts to get SlimeVR working on linux/with OpenVR.

So, current state of the art for slimevr + the open source openxr stack (monado/wivrn) is described at https://lvra.gitlab.io/docs/slimevr/. In short, there is both infrastructure in monado to use steamvr drivers, and a work in progress branch for having monado talk directly to slimevr via solarxr.

From what I remember, openxr was originally going to have a driver layer, but that never really materialized? So layers would be the way to do it if we wanted to do it generically, across any runtime, last I heard. I'm not up to date on whether this has been attempted, or if there are reasons this shouldn't be pursued.

The current "normal" way to do it is to just build support into the runtime, it seems.

On the feeder side of things, I've been itching to convert it into an openxr application for awhile. (I only really wanted to maintain a single API, which is why there's not a separate openxr-based feeder, or multi-api support, like wlx-overlay-s.) However, that doesn't seem compatible with steamvr's openxr implementation -- you can't mix an openxr-based overlay and openvr-based games, last I checked.

EDIT: this might be the same reason we never ended up with an openxr layer -- it likely wouldn't work with openvr games, so we'd need the openvr driver anyway. And other ecosystems either have their own ways around it (monado), or it's unclear how we would be able to add our layer to the system (standalone headsets, such as quest and pico).

For discussion (and finding people who are more in-touch on this point than I am): On the SlimeVR Discord, there is the #monado-fbt channel, and there is of course the Linux VR Adventures discord, in general.