Fix contact constraints being cleared many times with many backends (#…

…424)

# Objective

The `custom_collider` example is currently broken on some runs, because contact constraints are cleared by every instance of `generate_constraints`. If there are multiple collision backends at the same time, they can overwrite each other's results.

## Solution

Clear contact constraints in a separate system that always only has one instance.

src/collision/narrow_phase.rs

@@ -135,6 +135,18 @@ impl<C: AnyCollider> Plugin for NarrowPhasePlugin<C> {
         );
 
         if self.generate_constraints {
+            if is_first_instance {
+                // Clear contact constraints.
+                app.add_systems(
+                    self.schedule,
+                    (|mut constraints: ResMut<ContactConstraints>| {
+                        constraints.clear();
+                    })
+                    .after(NarrowPhaseSet::PostProcess)
+                    .before(NarrowPhaseSet::GenerateConstraints),
+                );
+            }
+
             // Generate contact constraints.
             app.add_systems(
                 self.schedule,
@@ -262,9 +274,6 @@ fn generate_constraints<C: AnyCollider>(
 ) {
     let delta_secs = time.delta_seconds_adjusted();
 
-    // Clear contact constraints.
-    constraints.clear();
-
     // TODO: Parallelize.
     for contacts in narrow_phase.collisions.get_internal().values() {
         let Ok([collider1, collider2]) = narrow_phase