[mediacapture-depth] Specialize ImageData type for Depth

bbernhar has just created a new issue for 
https://github.com/w3c/mediacapture-depth:

== Specialize ImageData type for Depth ==
Per discussion in https://github.com/w3c/mediacapture-depth/pull/130, 
this proposal suggests creating a specific `DepthImage` type where 
`DepthImage.data` creates a default `Uint16ClampedArray` view. In 
contrast to the ED 6.7, `ImageData.data` will not have an overloaded 
meaning (color vs depth) with the depth extension and equally (and 
more intuitively) access 16-bit dexels without further reconstruction 
(due to packing of D16 with RG88 for `video+canvas` sink). 
Implementation wise, the `DepthImage` object is exposes "view" of the 
internal `ImageData`. 

Alternatively, the `ImageData` object could be modified (or extended) 
but this approach frees us from handling depth-specific design issues 
with the canvas authors.

https://msdn.microsoft.com/en-us/library/windowspreview.kinect.depthframe.aspx

Please view or discuss this issue at 
https://github.com/w3c/mediacapture-depth/issues/131 using your GitHub
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