Implement more nodes

[vkoskiv]

Here's a big list to track where we're at. Progress is currently happening in the node-improvements branch. A lot of functionality is still written in the core logic that could be moved over to using nodes instead. I'll mark nodes that require refactoring with a * For the most part, I'm looking at the nodes available in the Cycles rendering engine to spot some easy nodes to start with.

• [x] texture node (TODO: Rename to color, perhaps)
• [x] Diffuse BSDF
• [x] Glass BSDF
• [x] Metal BSDF
• [x] (Dubious) Plastic BSDF (combines diffuse + glass)
• [x] Mix node
• [x] Grayscale converter node
• [x] Texture coordinate node (51ce446b2)
• [x] Fresnel node *
• [x] Transparent BSDF (Alpha blending) * (dda5c927191bba3fd8a806cc6a0b505d239b7a53)
• [x] Emission bsdf * (sort of implemented in f4d52e3d in a hacky way)
• [ ] Sky texture (Logic exists in sky.c, need to expose as a node)
• [ ] Environment map as an output node.
• [x] Add shader node
• [x] blackbody node
• [x] Value type
• [x] Vector type
• [x] Math node
• [x] Vecmath node