Normal maps

Normal maps modify the normals used to calculate lighting at a point on a face. Currently, every Object in pbr provides the [normal at a point](https://github.com/hunterloftis/pbr/blob/master/pkg/render/tracer.go#L36). There's [a place](https://github.com/hunterloftis/pbr/blob/master/pkg/surface/triangle.go#L99) in the `At` function to add a normal map value to the computed normal and return that instead.

Normal maps are already loaded in along with other textures, they just aren't used.

Adding normal maps will greatly enhance the detail and realism of surfaces by allowing surfaces that have texture.