The formula was not consistent across Blender and behaved strangely, now it is
a simple linear blend between color1 and min(color1, color2).
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D489
* Simplify default color values, where each component was the same.
* Initialize closures as Null Closure, rather than assigning an existing closure, gets overwritten anyways.
Previously the OSL Mix shader node was clamping the 2nd color component (green) to the 3rd color component (blue). Now every component is clamped on its own.
* Moved kernel/osl/nodes to kernel/shaders
* Renamed standard attributes to use geom:, particle:, object: prefixes
* Update stdosl.h to properly reflect the closures we support
* Fix the wrong stdosl.h being used for building shaders
* Add geom:numpolyvertices, geom:trianglevertices, geom:polyvertices attributes
