Newer hit test API and input event support

[takahirox]

This PR adds

• Newer Hit Test API support #73 #53
• Input event support #78
• Clear opaque framebuffer every frame #79

With this change, for example the following WebXR AR applications start to work

• WebXR samples HitTest example
• Three.js WebXR AR cones example
• Three.js WebXR AR Hit Test example
• Three.js WebXR AR paint example

(See #80 if you want to try Three.js examples.)

Note that this change breaks the examples in splash.html using old hit test API. We should update them in another PR.

Another note, currently it takes offset ray direction but doesn't take offset ray origin in this PR. As we discussed in slack, we may change to

• if it doesn’t pass through origin of viewer space, it’s invalid
• if it doesn’t pass through screen, it’s invalid. Can just test this using horizonal and vertical FoV’s

let's do them in another PR if needed because I don't want to make this PR further bigger.

[blairmacintyre]

@joshmarinacci I don't have time to review this, unfortunately.

[takahirox]

What are the tests that you are using to validate it?

I tested on the existing WebXR AR examples I listed in the PR comment. Let me know if any (automatic) tests we should validate.

[joshmarinacci]

looks good to me.

[joshmarinacci]

you can merge it now