Support changing the cursor and allowing interaction with some regions of the page when in "eyedropper mode"

[BoCupp-Microsoft]

In Photoshop, a user can select the eyedropper tool from a tool palette and then repeatedly click to set the current foreground or background color. When the user wants to change tools, they can hover back over the tool palette and the eyedropper cursor will change back into a regular pointer arrow and any clicks on a tool palette button will register as clicking the button instead of sampling a color.

The EyeDropper API proposal should support excluding some portions of the web page from color sampling to enable experiences like the one described above. A CSS property like eyedropper: exclude | auto; is probably sufficient.

If we add support for excluding regions of a web page, we should also be explicit about when it applies. One interesting example: when a document that isn't part of the EyeDropper's relevant document has a portion of its content excluded from "eyedropper mode" should that directive be honored?

[BoCupp-Microsoft]

Mentioning some interested parties: @ipopescu93 @inexorabletash @melanierichards.