Potentially incorrect wording in the specification

When going over the spec for issue #43, I have realized that we may have a mismatch between what the specification says, and what we do in our ARCore-backed implementation in Chrome.

Namely, the spec says that in the buffer that we return, "each entry corresponding to distance from the view's near plane to the users' environment".

ARCore's documentation seems to have a conflicting phrasing:

1. In ArFrame_acquireDepthImage(), we have "Each pixel contains the distance in millimeters to the camera plane".
2. In Developer Guide, we have "Given point A on the observed real-world geometry and a 2D point a representing the same point in the depth image, the value given by the Depth API at a is equal to the length of CA projected onto the principal axis".

If ARCore returns data according to 1), then I think it'd be acceptable to leave the spec text as-is, but then our implementation may not be correct (namely, I think we may run into the same issue that causes @cabanier to need to expose at the very least the near plane distance that ARCore internally uses?).

If ARCore returns data according to 2), then the values in the buffer we return are not going to depend on the near plane. In this case, we are not going to be compliant with the spec (we don't have a distance from near plane to user's environment), and the only way to be compliant will require us to adjust each entry in the buffer - this may be expensive given that this'll happen on CPU. IMO the best way to fix this would be to change the spec prose here, but I think this may be considered a breaking change, so we'll need to discuss how to move forward.

I'm going to try to confirm with ARCore what is actually their behavior, I'm not sure if this issue is actionable until that happens.

Start by comparing the depth-sensing specification wording with the linked ARCore documentation for ArFrame_acquireDepthImage() and depth values. Confirm which distance ARCore returns, then determine whether the implementation or specification prose requires a change; the issue is done when the behavior and compliant wording are agreed.