Light get world direction

Addresses the duplicate code in the renderers for figuring out the direction of the SpotLight and DirectionalLight by moving it into a function in the Light classes.

In response to @WestLangley's comments I have made a shared but unusable static function on Light that is then attached in a usable way to the prototypes of SpotLight and DirectionalLight.  I did this to avoid writing the same code twice.

Fixes #4258.