Mesh simplification library

I've released a mes simplification library here: https://github.com/nickgildea/ng_mesh

No external dependencies beyond the qef_simd.h single file library I released last week.

SSE2 QEF implementation

I've ported the existing QEF implementation to SSE2. You can grab the code here: https://github.com/nickgildea/qef/blob/master/qef_simd.h

It should be possible to just drop that in to your project as a replacement, assuming you then enable SSE2 extentions when building. I saw ~2.5x speed up in my tests.