Add const to quaternion conversion operators

[oliver-om]

The explicit conversion operators from qua<T, Q> to mat<3, 3, T, Q> and mat<4, 4, T, Q> can be const, because they do not modify the quaternion. Without this change, the following code does not compile:

const glm::quat q(0.0f, 0.0f, 0.0f, 0.0f);
const glm::mat3 R(q);