Add body tracking based on XR_BD_body_tracking

[gareth-morgan-nv]

Adds body tracking to the LOVR LUA interface. It follows the handTracking pattern and adds a body device that can be passed to getSkeleton:

    local skeleton = lovr.headset.getSkeleton('body')
    if skeleton then
      local pelvis = skeleton[1]   -- pelvis
      local head = skeleton[16]    -- head

Internally uses the XR_BD_body_tracking extension, and exposes the skeleton via its joint conventions, but could use any of the other OXR body tracking extensions:

An example LUA script using it is here:
https://gist.github.com/gareth-morgan-nv/5218bdb91b47ac956edea82b54fe063a

[bjornbytes]

Thanks for the PR, this is exciting!

• I'm curious about the reason you chose XR_BD_body_tracking over the other body tracking extensions.
• Have you tried this out on a specific device/runtime combination? I don't think I have access to any hardware that supports this extension atm, so it would be useful to know where it's been confirmed to work.
• I will leave some more feedback on the code, but upfront I will say that there are way, way too many comments, relative to the typical style used in the rest of the codebase. (Some comments are fine! But we try to reserve them for the most important things to call out, or to break up very long functions/files into sections).
• Minor note: I do think it's pretty awkward to have "devices" for body parts, e.g. elbow/left, chest, and now even body. I think in the future lovr.headset.getSkeleton will be changed to take a SkeletonType (either hand/left, hand/right, or body), and then all the body-related devices could be removed and folded into getSkeleton. But this is orthogonal to this immediate PR, and can happen in the future. There's some more discussion in Body Tracking #840.

[gareth-morgan-nv]

* I'm curious about the reason you chose `XR_BD_body_tracking` over the other body tracking extensions.

It's the hardware I had, also its a simpler extension that the other ones (no confidence, base pose, etc.)

* Have you tried this out on a specific device/runtime combination?  I don't think I have access to any hardware that supports this extension atm, so it would be useful to know where it's been confirmed to work.

We have confirmed this works with an experimental version of CloudXR

* I will leave some more feedback on the code, but upfront I will say that there are way, way too many comments, relative to the typical style used in the rest of the codebase.  (Some comments are fine!  But we try to reserve them for the most important things to call out, or to break up very long functions/files into sections).

I've done a pass removing them to match the rest of the code, does that look OK?

* Minor note: I do think it's pretty awkward to have "devices" for body parts, e.g. `elbow/left`, `chest`, and now even `body`.  I think in the future `lovr.headset.getSkeleton` will be changed to take a `SkeletonType` (either `hand/left`, `hand/right`, or `body`), and then all the body-related devices could be removed and folded into getSkeleton.  But this is orthogonal to this immediate PR, and can happen in the future.  There's some more discussion in [Body Tracking #840](https://github.com/bjornbytes/lovr/issues/840).

That makes sense.

[bjornbytes]

Thanks, merging!