Dirty Rectangles

Recently I've been looking for a way to optimize rendering in the engine.  A lot of ideas are out there, but the best one (by far) is "dirty rectangles".  This is something that will apply to the canvas context since it's typically wiped for each frame and completely redrawn.  I'm just starting on this so I don't expect to have anything for a little bit.  But, if this results in increased framerates (like I hope it will) it'll push the engine to a place where people can expect more from it.  As it stands now, a game is completely limited by how quickly a browser can redraw the entire canvas.