Rewrote vec3::operator+= and operator-= into inline void operator+=(vec3&, const vec3&)
Replaced vec3::operator*(array<size_t, 3>) by vec3 operator+(vec3&, array<size_t, 3>)
Inlined dot product vec3::operator*(vec3&)
Replaced vec3::zero by zero(array<float, 3>&)