Move the quad construction to the `procedural` module.
Add point rendering.
Add a basic scene graph + lots of little things.

At the moment, the scene graph only contains transformations and objects. Whenever a property of a
scene graph (other than its tranformation) is modified (eg. its color), the change is cascaded to
all its descendent objects.

* Backface culling can be activated/deactivated on a per-object basis.
* Wireframe rendering can be activated/deactivated on a per-object basis.
* Cylinders and cones are now procedurally generated.
* Add procedural generation of Bezier surfaces.
* Add a material `NormalsMaterial` to draw normals.
* See the `groups.rs` demo to add a group of objects to the scene graph.
* Add a `MaterialManager` to cache named materials.
* Add a `MeshManager` to cache named meshes.