Improve performance for model builder when adding vertex.

[RobEwbank1]

When building models in C++ using model builder it's does quite a bit of heavy lifting that in some cases it's not needed. I'd like to add a couple of ways to improve performance.

1. Optionally add a method to prevent the hash / map lookup to remove duplicated vertexs, not certain about the name. I think adding as a separate function rather than option on first is better as it prevents a branch.

VtxIdxType ModelBuilder::AddVertexFast(const Vertex &vtx) { auto &list = lists.back(); size_t idx = list.vtx.size(); list.vtx.push_back(vtx); return VtxIdxType(idx); }

2. Add a way to reserve the current vtx / idx in the list:
   void ModelBuilder::ReserveCurrentList(size_t size){ auto &list = lists.back(); list.vtx.reserve(size); list.idx.reserve(size); }
   This will prevent re-allocation in the vectors if you know in advanced how many verts you have.

Please let me know your thoughts