html5-canvas Compositing Erase existing shapes with "destination-out"
Example
context.globalCompositeOperation = "destination-out"
"destination-out" compositing uses new shapes to erase existing drawings.
The new shape is not actually drawn -- it is just used as a "cookie-cutter" to erase existing pixels.
context.drawImage(apple,0,0);
context.globalCompositeOperation = 'destination-out'; // bitemark erases
context.drawImage(bitemark,100,40);
