Make MouseJoint:getBodies() return only one body (fixes #1179)

Override Joint's Body getters in MouseJoint, to make the second (user-supplied) body the first body returned, and return nil for the second (internal) body

src/modules/physics/box2d/MouseJoint.cpp

@@ -91,6 +91,16 @@ float MouseJoint::getDampingRatio() const
 	return joint->GetDampingRatio();
 }
 
+Body *MouseJoint::getBodyA() const
+{
+	return Joint::getBodyB();
+}
+
+Body *MouseJoint::getBodyB() const
+{
+	return nullptr;
+}
+
 } // box2d
 } // physics
 } // love