Blender Git Loki

Cleanup: Move common CUDA/OptiX Cycles device code into separate file

This reduces code duplication between the CUDA and OptiX device implementations: The CUDA device
class is now split into declaration and definition (similar to the OpenCL device) and the OptiX device
class implements that and only overrides the functions it actually has to change, while using the CUDA
implementation for everything else.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D6814