- Fix Warning size_t int conversions
- Use bool instead of int for success

Fix crash for meshes with large faces. If the input adjacency is faulty, it gets recomputed internally (fixes #76)

Fields can't be constexpr if they are of an incomplete type.

Unnecessarily, the constructor required the inherited type GLMatrixT only
to initialize its parent type Matrix4x4T. Thus, we can loosen the type
requirement and simply require Matrix4x4T as the initializer.