Render Node to Image

It would be really useful to render any given node with all effects etc into a Image buffer. Including alpha for anywhere the node doesn't draw. We already must be able to do this internally to implement Node.cache = true.

There are three main use cases for this:

(1) In tools/apps where I want icon versions or zoomed version of some nodes.
(2) For performance where I am rendering a expensive set of nodes many times, ie colorful circles.
(3) Where I want to save out the nodes to a image file like JPEG or PNG. This will also require fx image to Java2D BufferedImage conversion API or a Image save API.