Converted Renderer to singleton - saves on graphics resources by reusing
a single pool.
Changed Renderer calling convention within GameScene

Changed all public static getters in Singletons to public synchronized
static - protect from possible non java-fx thread calls.

Also hid all get() methods in Singletons - convenience methods are
available

Engine, Cache, Audiologist and Renderer are Singletons 

Todo: convert SceneGraph to singleton.