Add Scene prop projectPointerEventRef

[gilbox]

Previously, I was adding ref prop to Scene component ie ref={sceneRef => this.scene = sceneRef}. In order to get direct access to projectPointerEvent function.

... But since you refactored some stuff I can no longer access the projectPointerEvent function. This change allows me to do this:

projectPointerEventRef={projectPointerEvent => this.projectPointerEvent = projectPointerEvent}

[Izzimach]

I'll merge this so that your stuff works, but props may move around eventually to break this.