precompute new indexbuffer
- new vertexbuffer first contains vertex points in order of original vertices in vertex buffer
- new vertexbuffer then contains edge points that were created in order of edgeIndices

This will make it easier to iterate over all meshes in subdivide when we
want to add that feature.

vertexIndices: Array of single indices with array with offsets instead of one array with index-vectors

Also compute duplicate edge points for second traingle.