Texturing Objects

[Tloru]

Right now, objects can only take on solid colors. This is not a limitation in the render itself, it's perfectly capable of building textured objects.

To implement this feature, two things need to be done. First, traits.rs should be refactored to include a textured trait, and the material function should be moved out of March and Trace. All objects should be required to implement the material trait.

Additionally, the material function should be refactored to take a 3d point, so the texture can be determined.