Changed TriangleMesh::materials_ to be a std::vector<std::pair<std::string, Material>> so that the order of materials when loading a mesh is respected.
Using the unordered map caused issues when a Default Material and a texture were loaded and the texture would end up as the second material in the iterator.
Now TriangleMesh::triangle_material_ids_ will indicate what material index should be used.
A warning is given when passing a TriangleMesh to the visualizer and more than one material index is found, and the minimum value found in TriangleMesh::triangle_material_ids_ is used.
If no triangle_material_ids_ exist then the first material in the order they are loaded will be used.