In the process, find and correct a misunderstanding about vertex
attributes. Here's my current understanding. Vertex attributes are
program-independent. When you make a draw call, every enabled attribute
has to have a vertex buffer object bound to it. A simple way to make
sure this happens is to enable only the attributes used by the active
program.
For now, this is just a thin wrapper around the old element structure,
which was renamed to `Sphere` in the previous commit. The biggest
organizational change is moving `cast` into the `DisplayItem` trait.
This makes way for an `Element` trait. Some `Sphere` variables, like the
arguments of the sphere insertion methods, have been renamed to show
that they refer specifically to spheres. Others, like the argument of
`ElementOutlineItem`, have kept their general names, because I expect
them to become `Element` trait objects.
