Added overdraw visualization.

Profiled using
```
  var startTime = wtf.now();
  ...
  var endTime = wtf.now();
  goog.global.console.log('duration: ' + (endTime - startTime));
```
On my work desktop:
1024x768 context: 10-30ms
1920x1200 context: 35-55ms

On my work chromebook:
1024x768 context: 30-40ms

If/when I get around to adding options, turning off this calculation would be a good option to add.