API for extracting hdr (etc?) metadata from images and videos

[kdashg]

Related to #36, there is a usecase for a page to be able to manipulate an image or video in a canvas, and then have the canvas inherit the hdr metadata from the image/video.
One way to do this is to, along with an API to assign hdr metadata (as in #36), have an API for extracting hdr metadata from an image/video.