Allow btFixedConstraint to constrain rigid bodies against the world.

[AccessViolationEnjoyer]

Other constraints, such as btHingeConstraint, btPoint2PointConstraint and btSliderConstraint, all have constructors that take a single rigid body. This is missing from btFixedConstraint, although it is easy to add:

btFixedConstraint::btFixedConstraint(btRigidBody& rbA, const btTransform& frameInA)
:btGeneric6DofSpring2Constraint(rbA, frameInA)
{
    setAngularLowerLimit(btVector3(0, 0, 0));
    setAngularUpperLimit(btVector3(0, 0, 0));
    setLinearLowerLimit(btVector3(0, 0, 0));
    setLinearUpperLimit(btVector3(0, 0, 0));
}