Add InstancedMesh2 Culling behavior for shadow casting

[johnnyrainbow]

Hey, I love the instancedMesh2 implementation, its fantastic.

However i'd very much like the option to add a culling behaviour that works similar to CullingDynamic, however only culls objects that are both offscreen AND not casting a shadow on screen. (example, a tree outside the frustrum, casting a shadow on a plane. The visible within frustrum shadow disappears when the out of frustrum object is culled. I'd like to prevent culling if the shadow is in frustrum.

Would this be possible?