I want the ability to draw a node (or graph of nodes) into a buffer. This can currently be done by calling paint on the underlying SGNode with a BufferedImage's graphics context. I'd like a real API for this that can be easily hardware accelerated.

requirements:
* paints a node into an image. can paint a graph of nodes by nesting nodes inside of a group
* not required to paint a whole scene into an image
* not required to paint nodes which are already in a scene. can be restricted only to headless nodes
* can draw into the image multiple times, compositing against current contents
* can set a blend mode to use when compositing against current contents
* can be cleared
* does not allow direct access to the pixels (no getPixel(x,y) function), this allows for better hw acceleration since all operations can be done in the card with no texture readbacks.
* images should be independent so we can swap them in and out of ImageViews (for use with creating live trails and other time based effects)

API proposal:

javafx.scene.image.WriteOnlyImage extends Image {
public-init var width:Integer;
public-init var height:Integer;
// make fully transparent. erase all pixels.
public function clear():Void;
// draw node into buffer
public function draw(node:Node):Void;
// draw node using blend mode
public function draw(node:Node, mode:BlendMode):Void;
}

example usage:

var nodes = Group { content: [ Rectangle { }, ImageView{image:Image{}}, Circle{} ] };
var buffer = WriteOnlyImage { width: 100 height: 100 };
buffer.draw(nodes);
var view = ImageView { image: buffer };

Stage {
scene: Scene {
content: view
}
}