Change C++ interface to convert indices
Change C++ interface to convert indices

This allows us to expose entrypoints for just 32-bit indices which
simplifies conversion to C interface and reduces the surface area.

In general 16-bit indices seem like an odd case - a typical mesh
pipeline would work with 32-bit indices and only convert to 16-bit
indices at the very end, but we still provide convenience templated
functions.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer