Added overdraw visualization.

Some code, such as this function, is duplicated between ``HighlightVisualizer`` and ``OverdrawVisualizer``. This stems from both using ``OverdrawSurface`` and displaying overdraw colors.

* ``this.mutators['WebGLRenderingContext#clear']`` is identical in each
* ``createSurface`` has one extra argument to a constructor
* ``createProgram`` replaces 'overdraw' with 'highlight' in three names, but creates the same program
* ``handleDrawCall`` is identical, except for which of 'overdraw' or 'highlight' is passed to a function.

While this duplicate code could go in ``DrawCallVisualizer``, it really only applies to Visualizers that need to show overdraw information. I could rename ``OverdrawVisualizer`` and make ``OverdrawVisualizer`` a new abstract class between those two classes and ``DrawCallVisualizer``.

Thoughts on class names? Should I restructure this?