Added overdraw visualization.

If Visualizers skip pre/post event handlers when not active, this extra seek is required to setup surfaces, create programs, etc. by completing a run through the playback with this ``Visualizer`` active.

In most cases, highlight and overdraw will both seek from the beginning anyway, but this is a bit confusing right now.

If ``if (!this.active) return;`` were moved from ``handlePostEvent`` in ``Visualizer`` to ``handleDrawCall`` in ``OverdrawVisualizer`` and ``HighlightVisualizer``, this confusing seek call could be avoided.