VisualizationView improvements and bug fixes.

* Fixes race condition bugs that can lead to NPEs.
* Changes Layer interface to hand in VisualizationView everywhere. This cuts down on the number of parameters to methods like onStart().
* Adds new methods to Layer interface to make it easier to preload assets like fonts.
* Adds TextShape and PoiShape.
* Fixes bug in OccupancyGridLayer. The texture was not unbound after rendering.
* Changes XYOrthographicCamera to separate the ROS to screen transform and the camera transform. This was necessary to enable rendering text aligned with the screen.