Fixes and refactors following code review.

Changes include:

* Added Visualizer suffix to Highlight and Overdraw.
* Pass native objects instead of dictionaries.
* Cleaned up OffscreenSurface initialization.
* Refactored IVisualizer interface into Visualizer abstract class.
* Refactored Visualizer usage within Playback, avoids extra closures.
* Visualizers now use pre and post event handlers.
* Fixed a color conversion issue for overdraw calculation.
* Added more colors to the overdraw visualization.
* Cleaned up some type annotations, comments, and names.