Introduce ability to start and stop depth processing

[alcooper91]

Introduces the depthActive property to allow pages to request starting or stopping the depth activity.

Addresses #52

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Preview | Diff

[cabanier]

This is a bit weird. I don't think we have logic in WebXR that triggers by setting a boolean.
Maybe this would be more suitable if we add it to the renderstate?

[alcooper91]

We don't have a boolean, but we do have a float that functions very similarly: https://immersive-web.github.io/webxr/#dom-xrwebgllayer-fixedfoveation

I think there was some opposition to adding it to the renderState. Mainly @toji IIRC?

The original proposal was a pair of methods; but I felt that this was more elegant since calling the start/stop methods didn't necessarily guarantee anything.

[alcooper91]

@cabanier I've shifted away from the attribute.

[alcooper91]

/agenda discuss options to start/stop depth sensing

Since the in-comment-thread tag didn't seem to flag it.

[AdaRoseCannon]

Thanks, interesting that probot doesn't get the comment events from inline code discussions.

[alcooper91]

I did make one rename here from (start|stop)Depth -> (start|stop)DepthSensing to make the methods a little bit clearer. Otherwise, this should now address all of the feedback from yesterday's meeting to the best of my ability.

[toji]

LGTM, thanks! One minor comment.

[alcooper91]

@bialpio I'll merge tomorrow AM unless you provide additional feedback.