SceneManager manages the platform/framework-controlled part of the global Scenic scene
graph, with the fundamental goal of connecting the physical display to the product-defined user
shell. The part of the scene graph managed by the scene manager is split between three Flatland
instances, which are linked by view/viewport pairs.
ScreenCoordinates represents a point on the screen. It can be created from pixels, pips, or
millimeters and the coordinate vales can be retrieved in any of those units.
A SceneManager manages a Scenic scene graph, and allows clients to add views to it.
Each SceneManager can choose how to configure the scene, including lighting, setting the
frames of added views, etc.