feat: implement new "small-steps" solver + joint improvements

src/geometry/contact_pair.rs

@@ -283,7 +283,7 @@ pub struct ContactManifoldData {
 pub struct SolverContact {
     /// The index of the manifold contact used to generate this solver contact.
     pub(crate) contact_id: u8,
-    /// The world-space contact point.
+    /// The contact point in world-space.
     pub point: Point<Real>,
     /// The distance between the two original contacts points along the contact normal.
     /// If negative, this is measures the penetration depth.

src/geometry/narrow_phase.rs

@@ -927,6 +927,7 @@ impl NarrowPhase {
 
                 for manifold in &mut pair.manifolds {
                     let world_pos1 = manifold.subshape_pos1.prepend_to(&co1.pos);
+                    let world_pos2 = manifold.subshape_pos2.prepend_to(&co2.pos);
                     manifold.data.solver_contacts.clear();
                     manifold.data.rigid_body1 = co1.parent.map(|p| p.handle);
                     manifold.data.rigid_body2 = co2.parent.map(|p| p.handle);
@@ -944,10 +945,13 @@ impl NarrowPhase {
 
                         if contact.dist < prediction_distance {
                             // Generate the solver contact.
+                            let world_pt1 = world_pos1 * contact.local_p1;
+                            let world_pt2 = world_pos2 * contact.local_p2;
+                            let effective_point = na::center(&world_pt1, &world_pt2);
+
                             let solver_contact = SolverContact {
                                 contact_id: contact_id as u8,
-                                point: world_pos1 * contact.local_p1
-                                    + manifold.data.normal * contact.dist / 2.0,
+                                point: effective_point,
                                 dist: contact.dist,
                                 friction,
                                 restitution,