VRaySunSky : Added "Improved" sky model

VRaySunSky : Added albedo blend angle and horizontal offset and regroup all albedo options in a new Ground Albedo rollout.

VRayRenderElement (LightSelect) : Added Light Path Expressions (LPE)

VRayRenderElement (LightSelect) : Added Indirect and Subsurface light select mode

VRayMtl : Added Coat layer

VRayMtl : Added Sheen layer

VRayMtl : Added Metalness

VRayEnvironmentFog : Added colored transparency

VRayPluginMtl : Implemented VRayPlugin-based generic material node with automatic user interface creation based on plugin description