Updated WebXR samples incoming!

(The following is copied from the PR description of the just-posted
https://github.com/immersive-web/webxr-samples/pull/37 Please leave any
comments regarding the updated samples under that link.)

Hey all!

While we've been working on updating the WebXR spec I've been quietly
working on a new and improved version of the sample pages that both updates
them to match the VR complete version of the spec and makes a variety of
improvements to the code to hopefully make the samples easier to maintain
and read.

The main changes you'll see are:

   - All samples updated to conform to the latest WebXR spec
   - Samples run on Chrome 76+'s WebXR implementation (via a minor shim)
   and any WebVR-compatible browsers via the most recent polyfill work by
   @jacobcdewitt <https://github.com/jacobcdewitt>.
   - Running with the polyfill is now the main mode, with a link to
   explicitly run without the polyfill provided.
   - New samples for gamepad state and teleportation, based on samples
   built to test Chrome by @klausw <https://github.com/klausw> and
   @jacobcdewitt <https://github.com/jacobcdewitt>.
   - Click-and-drag viewing for all inline samples.
   - General code cleanup to try and make reading the samples a bit easier.
   - The "Cottontail" library has been renamed to simply "renderer" (to
   prevent confusion).
   - All samples use JS modules to import the renderer and other utility
   libraries now, eliminating the previous build step required to use them.
   (All modern browser support JS modules at this point).

This "PR" is, well, pretty damn huge. I don't actually expect anyone to
review it in the traditional sense because it would be a massive time sink
and it's unlikely you'll catch many issues from simply staring at entire
files worth of changes. Instead, I've mirrored these changes to
https://toji.github.io/webxr-samples and I'd like to request that you take
the time to try the new versions of the samples out on your browser and
device pairing of choice to see if you run into any unexpected issues.

Assuming nothing catastrophic comes up, I'm going to plan on merging this
PR within a week, so that the samples page is live and ready to go when we
cut the VR complete working draft and I start asking developers to take a
look.


Thanks!

--Brandon Jones