SteamVR tracker support

[avaer]

We would like to get generic Vive trackers (not wands) exposed as gamepads in navigator.getGamepads.

We already support the querying infrastructure needed; we just need to hook in the case of generic trackers into the render loop.

[avaer]

The native side of this is here: https://github.com/webmixedreality/exokit/blob/328f5274400cf736ce04c250290770bdb27f18e9/deps/openvr/src/ivrsystem.cpp#L783

The JS side would be here: https://github.com/webmixedreality/exokit/blob/328f5274400cf736ce04c250290770bdb27f18e9/src/index.js#L1238

[kfarr]

This is a screenshot of how Unity handles mapping input from SteamVR API

In a perfect world, as an A-Frame application developer, I can target a specific tracker and mapping, for example:
<a-entity tracked-controls="id: OpenVR Tracker Left Foot"></a-entity>

It is up to the user to determine mapping via existing SteamVR interface, but the above helps a developer target that mapping.

Note that the A-Frame tracked-controls component allows for querying controllers with idPrefix as well as id to provide backwards support such that an application that has idPrefix: OpenVR Tracker won't break if there is a change in behavior to append to the pose role string at the end of the current controller name (id) string.