Fix crash in broad-phase after shape modification.

Fix #180 and #177

src/geometry/broad_phase_multi_sap/broad_phase.rs

@@ -369,6 +369,7 @@ impl BroadPhase {
                     // We need to promote the proxy to the bigger layer.
                     layer_id = self.ensure_layer_exists(new_layer_depth);
                     self.proxies[*proxy_index].layer_id = layer_id;
+                    self.proxies[*proxy_index].layer_depth = new_layer_depth;
                 }
             }
 

src/geometry/broad_phase_multi_sap/sap_region.rs

@@ -90,7 +90,11 @@ impl SAPRegion {
     ///
     /// Returns `true` if this region contained the proxy. Returns `false` otherwise.
     pub fn proper_proxy_moved_to_a_bigger_layer(&mut self, proxy_id: SAPProxyIndex) -> bool {
-        if self.existing_proxies[proxy_id as usize] {
+        if self.existing_proxies.get(proxy_id as usize) == Some(true) {
+            // NOTE: we are just registering the fact that that proxy isn't a
+            //       subproper proxy anymore. But it is still part of this region
+            //       so we must not set `self.existing_proxies[proxy_id] = false`
+            //       nor delete its endpoints.
             self.subproper_proxy_count -= 1;
             true
         } else {