Use packet intersector for primary rays on GPUs
Use packet intersector for primary rays on GPUs

To run our packet intersector we need a local workgroup size of 64. CPU
devices only support a local workgroup size of 1. For CPU devices we
fallback to the standard individual ray intersector.

achilleasa/go-pathtrace,achilleasa/go-pathtrace,achilleasa/go-pathtrace