Nanogui previously relied on the Enoki library for basic vector and
matrix types (and before that, Eigen was used). At this point, Enoki has
become large project in its own right, targeting fairly advanced use
cases such as automatic differentiation on NVIDIA GPUs. Since NanoGUI
merely used a few trivial types (Vector2i/f, and Matrix4f), I decided to
simply provide basic implementations of these types here to make the
project more self-contained.

The vector types here continue to be interoperable with Enoki and e.g.
support implicit conversions.