Added overdraw visualization.

(in theory) you should not need this typecast - the compiler should figure out that the literal is of the type. If it doesn't work for property sets like this, then you could get around this by defining a registerMutator(name, mutator) on Visualizer, as I'm pretty sure it would work there.
```
  this.registerMutator('WebGLRenderingContext#clear', {
    post: function(visualizer, gl, args) {
      //...
    }
  });
```
Not too bad. You could also move all of these to a function like `setupMutators_` so that it wasn't in the constructor.