REFAC: Extensible Render Delegates

Adds the ability to utilize a custom/subclassed RenderDelegate.

This allows one to extend the rendering functionality by providing their own RenderDelegates & VisualModels. Examples:

• One example provided in this MR is an example side render delegate that gives graph rendering functionality.

Other possible examples:

• Customer shaders for VisualModels.
• Other VTK mappers such as 3d text, 2d text, billboarding.

Another benefit is you may develop these in their own projects & easily merge upstream if applicable.

Todo:

• Fix Bug in the examples RenderDelegate which causes the axes to flip.
• Retarget example. Remove some commented code & rename example.