[mediacapture-depth] Pull Request: Fix #114: Add depthNear and depthFar constrainable properties

anssiko has just submitted a new pull request for 
https://github.com/w3c/mediacapture-depth:

== Fix #114: Add depthNear and depthFar constrainable properties ==
As a fix to #114 this PR adds `depthNear` and `depthFar` constrainable
 properties. Note we switch to use meter as the unit (from 
millimeters) for the near value and far value as suggested by 
@martinthomson, and also the recovered depth measurement `d` is now 
reported in meters for consistency.

I have one question for the group:

* Do we want to special case `depthFar < depthNear`, or leave it to 
the implementation to figure out what to do in such a case?

Seeking review from @huningxin @martinthomson @jan-ivar @alvestrand et
 al.

See https://github.com/w3c/mediacapture-depth/pull/118