void Camera::rotate(const QQuaternion& q)
{
    Q_D(Camera);

    d->m_upVector = q.rotatedVector(d->m_upVector);
    d->m_cameraToCenter = q.rotatedVector(d->m_cameraToCenter);
    d->m_viewCenter = d->m_position + d->m_cameraToCenter;

    d->m_viewMatrixDirty = true; // <- here
}

void Camera::rotateAboutViewCenter(const QQuaternion& q)
{
    Q_D(Camera);

    d->m_upVector = q.rotatedVector(d->m_upVector);
    d->m_cameraToCenter = q.rotatedVector(d->m_cameraToCenter);
    d->m_position = d->m_viewCenter - d->m_cameraToCenter;

    d->m_viewMatrixDirty = true; // <- here
}