SteamVR Tracker support

[kfarr]

(not sure if this is the right location for this ticket so please inform me if it needs to be moved)

I am encountering difficulty in implementing HTC Vive Trackers in the current WebVR 1.1 standard. I'm curious how this is proposed to be supported in WebXR.

Here are the issues:

• WebVR 1.1 browsers appear to expose controllers (and trackers) as an array accessible through Navigator.getGamepads()
• There is inconsistency with how WebVR 1.1 browsers expose the trackers:
  -- Firefox sometimes exposes them, sometimes they are assigned handedness, sometimes not, unclear pattern to this behavior
  -- Exokit only explicitly exposes vive wands, does not expose trackers (related ticket SteamVR tracker support exokitxr/exokit#884)
  -- Supermedium has similar behavior to Firefox
• Further complicating things, Valve changed the behavior around managing inputs approx Oct 3 2018 during this release (search for "tracker") https://steamcommunity.com/games/250820/announcements/detail/3213772926319522701
• In Oct 10 2018 Valve introduced a built-in end-user-accessible SteamVR tool for Vive Tracker Mapping. This allows the end-user to map a "role" to a tracker such as "Left Foot" or "Right Hand". (this also created confusion which led to them to release a blog post about these changes)
• WebVR 1.1 browsers do NOT appear to pass the controller "role" that is set by the end-user using the new SteamVR Input Mapping configuration, making it impossible for WebVR applications to use specific trackers with reliability aside from random and/or rewriting a temporary mapping tool within the web application.

The ask for WebXR spec is to expose "role" that controllers may have as determined by SteamVR that allow targeting specific trackers from a web application.

[kfarr]

To help figure out how to map this, here are the possible values for controller "role" currently accessible from Unity using official SteamVR component:

There is still ongoing debate about how this should be handled at the SteamVR/OpenVR layer:
ValveSoftware/openvr#962

Here is a nice blogpost from the SteamVR team explaining how input / controller management was a big overhaul for SteamVR 2.0 in September 2018:
https://steamcommunity.com/games/250820/announcements/detail/1696059027982397407