Impulsive joint having issue with the T12 example when flipped

[soraxas]

After adding the support of dae via PR #732, and changing the example in https://github.com/dimforge/rapier/blob/master/examples3d/urdf3.rs to

diff --git a/examples3d/urdf3.rs b/examples3d/urdf3.rs
index 815cfb6..19dd423 100644
--- a/examples3d/urdf3.rs
+++ b/examples3d/urdf3.rs
@@ -24,7 +24,7 @@ pub fn init_world(testbed: &mut Testbed) {
     };
 
     let (mut robot, _) =
-        UrdfRobot::from_file("assets/3d/T12/urdf/T12.URDF", options, None).unwrap();
+        UrdfRobot::from_file("assets/panda/urdf/panda_relative.urdf", options, None).unwrap();
 
     // The robot can be inserted using impulse joints.
     // (We clone because we want to insert the same robot once more afterward.)

with the popular panda arm (asset uploaded here: panda.tar.gz), we can then obtain the following:

which seems to be upside down. I'm not 100% sure why, but that might be due to bevy being Y-up and with left-hand cooridnate system:

---

After further modifying the code to rotate the other way:

diff --git a/examples3d/urdf3.rs b/examples3d/urdf3.rs
index 815cfb6..89d084c 100644
--- a/examples3d/urdf3.rs
+++ b/examples3d/urdf3.rs
@@ -19,7 +19,7 @@ pub fn init_world(testbed: &mut Testbed) {
         create_colliders_from_collision_shapes: false,
         make_roots_fixed: true,
         // Z-up to Y-up.
-        shift: Isometry::rotation(Vector::x() * std::f32::consts::FRAC_PI_2),
+        shift: Isometry::rotation(Vector::x() * -std::f32::consts::FRAC_PI_2),
         ..Default::default()
     };

we can then obtain the following, which seems all good.

---

The following is the actual bug report, where, after all the previously described modification (and the reason for the inverted rotational vector), the original example with T12 looks like this:

The impulsive joints version seems to have some issue when the model is inserted with the shift of Isometry::rotation(Vector::x() * -std::f32::consts::FRAC_PI_2).