Math: explicit one-parameter Point*D constructor.

Was causing improper implicit conversions, such as here (example
directly from unit tests, where it was unintentionally used):

    Vector3 normal;
    Matrix4 transformation;
    auto transformedNormal = transformation*normal;

Not only that it was possible to multiply 4x4 matrix with 3-component
vector, but the resulting type was Point3D which was absolutely
confusing. Currently it must be explicitly converted:

    transformedNormal = transformation*Point3D(normal);