[mediacapture-screen-share] Provide a means to select only part of a screen to capture (#105)

guest271314 has just created a new issue for https://github.com/w3c/mediacapture-screen-share:

== Provide a means to select only part of a screen to capture ==
Firefox provide a "Take a Screenshot" feature which allows the user to select only a portion of a screen. That option should be provided for `getDisplayMedia()` in the form of constraints where specific coordinates can be passed; e.g., using `.getBoundingClientRect()` in the form of `{topLeft:<pixelCoordinate>, topRight:<pixelCoordinate>, bottomLeft:<pixelCoordinate>, bottomRight:<pixelCoordinate>}`, or at the selection UI, similar to how Firefox implements "Take a Screenshot" feature. Use case: The user only wants to share a specific element, e.g. a `<video>`. 

Please view or discuss this issue at https://github.com/w3c/mediacapture-screen-share/issues/105 using your GitHub account