Blender Git Loki

Cycles: Refactor kernel globals localization used for CPU

Move all resource management to a dedicated class. Reduces code
duplication between rendering and shading, as well as makes resource
management more localized and robust.

Additionally, removed the kgbuffer, which seemed to be some sort of
part of a finished project.

Should become handy when adding actual execution logic to the device
queue (which would need to have KernelGlobals in order to invoke
kernels).

Currently no functional changes. Tested by switching Session::render()
back to its operable state.