For the ambient light model, integers need to be remapped to a range of
`-1.` through `1.`. Add the `+` and `-` operators to `VectorN` to make
it a bit easier to normalize 4 values at once.
We now dereference the pointer given to us before adding the arguments
to an active list. This also factors out the switching logic from the
API wrappers, which helps us with a future commit where we autogenerate
all API wrapper functions.
Each texture unit now has its own texture transformation matrix stack.
Introduce a new texture unit configuration that is synced when changed.
Because we're no longer passing a silly `Vector` when drawing each
primitive, this results in a slightly improved frames per second :^)
