add "scan line" and "grid" overlay effects to livecodelab

[davidedc]

adding a "scanlines" or "grid" overlay effect should be easy and performant in both the canvas and webgl version.

An example of the "grid" overlay effect (although a faint one) see this: http://www.webgl.com/wp-content/uploads/2012/04/webgl-daftunes-2.jpg

One could start with implementing the canvas version, which consists with just an extra div with a repeating line or dot pattern. If that performs well in all browsers then that's it.

For webgl, in theory this stuff is better done via shaders, so one could look into that option as well if it makes things faster for webgl-enabled browsers.