Blender Git Loki

Cycles: Refactor storing of the integration result

This commit refactors how the integration result is stored: Instead of summing up
the PathRadiance in the integration function and returning the final color, the integration
function now fills a PathRadiance passed to it and just returns the alpha value.
The main kernel function then passes that PathRadiance to kernel_write_result, a new function
which then handles summing, clamping and storing of light and combined passes.

This commit by itself shouldn't change existing behaviour, but is needed for the upcoming
selective denoising.