feat: persistent islands + manifold reduction (#895)

* feat: initial implementation of contact manifold reduction

* feat: try bepu-like manifold reduction

* feat: simplification of the constraints counting and indexing logic

* feat: add concept of incremental islands with a single awake island

More islands manager fixes

* feat: start adding support for multiple awake islands

* feat: add more timings

* feat: implement incremental island split & merge

* chore: refactor islands manager into multiple files

* chore: refactor manifold reduction to its own file + add naive reduction method

* feat: add islands manager validation checks

* fix various bugs in the new islands system

* chore: remove redundant active_set_offset field

src/counters/collision_detection_counters.rs

@@ -8,6 +8,9 @@ pub struct CollisionDetectionCounters {
     pub ncontact_pairs: usize,
     /// Time spent for the broad-phase of the collision detection.
     pub broad_phase_time: Timer,
+    /// Time spent by the final broad-phase AABB update after body movement to keep
+    /// user scene queries valid.
+    pub final_broad_phase_time: Timer,
     /// Time spent for the narrow-phase of the collision detection.
     pub narrow_phase_time: Timer,
 }
@@ -18,6 +21,7 @@ impl CollisionDetectionCounters {
         CollisionDetectionCounters {
             ncontact_pairs: 0,
             broad_phase_time: Timer::new(),
+            final_broad_phase_time: Timer::new(),
             narrow_phase_time: Timer::new(),
         }
     }
@@ -26,6 +30,7 @@ impl CollisionDetectionCounters {
     pub fn reset(&mut self) {
         self.ncontact_pairs = 0;
         self.broad_phase_time.reset();
+        self.final_broad_phase_time.reset();
         self.narrow_phase_time.reset();
     }
 }
@@ -34,6 +39,7 @@ impl Display for CollisionDetectionCounters {
     fn fmt(&self, f: &mut Formatter) -> Result {
         writeln!(f, "Number of contact pairs: {}", self.ncontact_pairs)?;
         writeln!(f, "Broad-phase time: {}", self.broad_phase_time)?;
+        writeln!(f, "Final broad-phase time: {}", self.final_broad_phase_time)?;
         writeln!(f, "Narrow-phase time: {}", self.narrow_phase_time)
     }
 }