Introduce the PhysicsHook trait used for both contact filtering and contact modification.

src/geometry/contact_pair.rs

@@ -9,7 +9,10 @@ bitflags::bitflags! {
     pub struct SolverFlags: u32 {
         /// The constraint solver will take this contact manifold into
         /// account for force computation.
-        const COMPUTE_IMPULSES = 0b01;
+        const COMPUTE_IMPULSES = 0b001;
+        /// The user-defined physics hooks will be used to
+        /// modify the solver contacts of this contact manifold.
+        const MODIFY_SOLVER_CONTACTS = 0b010;
     }
 }
 
@@ -104,6 +107,8 @@ pub struct ContactManifoldData {
     /// The contacts that will be seen by the constraints solver for computing forces.
     #[cfg_attr(feature = "serde-serialize", serde(skip))]
     pub solver_contacts: Vec<SolverContact>,
+    /// A user-defined piece of data.
+    pub user_data: u32,
 }
 
 /// A contact seen by the constraints solver for computing forces.
@@ -165,6 +170,7 @@ impl ContactManifoldData {
             solver_flags,
             normal: Vector::zeros(),
             solver_contacts: Vec::new(),
+            user_data: 0,
         }
     }
 
@@ -205,9 +211,3 @@ impl ContactManifoldData {
     //     manifold.data.warmstart_multiplier = Self::min_warmstart_multiplier()
     // }
 }
-
-/// A contact manifold that can be modified by the user.
-pub struct ModifiableContactManifold<'a> {
-    manifold: &'a super::ContactManifold,
-    solver_contacts: &'a mut Vec<SolverContact>,
-}