Blender Git Loki

Cycles: Expose adaptive sampling filters via CPUKernels

The change includes moving some lines around, to arrange kernels
in a more clear and grouped way.

The CPUFunction invokation had to be modified to return auto type.
This way it's possible to return value from the kernel. It compiles
here with GCC. Didn't check whether this is something from newer
C++ standards. If so, we can do some declspec magic to deduct the
return type from the function type.

On one hand this is an extra indirection, but on another hand this
brings the following benefits:

- Benefit from microarchitecture optimizations.
- Exposes all kernels in a similar manner.