Should make new things more discoverable, avoid confusion when a
documented API isn't there and reduce the need for maintaining multiple
separate versions of the docs.
It allocates one more array, but that speeds up the calculation to twice
as fast. Before the benchmark was around 1 ms for flat normals and 12 ms
for smooth, now it's 6 ms for smooth. There is probably more I could do
(I feel like I could save at least one more normalization), however this
is good enough for now.
Also move it to a new GenerateNormals.h header so we can easily add
generateSmoothNormals() to it. The old API and header is deprecated and
will be removed in the future. I can't be bothered rewriting the old
code using the new thing, so it's preserved there as a mausoleum until
it gets finally nuked from the orbit.