Share the bvh of btBvhTriangleMeshShape's when possible

Results in decent cell-loading speed up. (only affects bullet versions < 2.83, since we use btScaledBvhTriangleMeshShape for >=2.83)

components/resource/bulletshape.cpp

@@ -62,7 +62,16 @@ btCollisionShape* BulletShape::duplicateCollisionShape(btCollisionShape *shape)
         // work around btScaledBvhTriangleMeshShape bug ( https://code.google.com/p/bullet/issues/detail?id=371 ) in older bullet versions
         btTriangleMesh* oldMesh = static_cast<btTriangleMesh*>(trishape->getMeshInterface());
         btTriangleMesh* newMesh = new btTriangleMesh(*oldMesh);
-        TriangleMeshShape* newShape = new TriangleMeshShape(newMesh, true);
+
+        // Do not build a new bvh (not needed, since it's the same as the original shape's bvh)
+        bool buildBvh = true;
+        if (trishape->getOptimizedBvh())
+            buildBvh = false;
+        TriangleMeshShape* newShape = new TriangleMeshShape(newMesh, true, buildBvh);
+        // Set original shape's bvh via pointer
+        // The pointer is safe because the BulletShapeInstance keeps a ref_ptr to the original BulletShape
+        if (!buildBvh)
+            newShape->setOptimizedBvh(trishape->getOptimizedBvh());
 #endif
         return newShape;
     }

components/resource/bulletshape.hpp

@@ -60,8 +60,8 @@ namespace Resource
     // Subclass btBhvTriangleMeshShape to auto-delete the meshInterface
     struct TriangleMeshShape : public btBvhTriangleMeshShape
     {
-        TriangleMeshShape(btStridingMeshInterface* meshInterface, bool useQuantizedAabbCompression)
-            : btBvhTriangleMeshShape(meshInterface, useQuantizedAabbCompression)
+        TriangleMeshShape(btStridingMeshInterface* meshInterface, bool useQuantizedAabbCompression, bool buildBvh = true)
+            : btBvhTriangleMeshShape(meshInterface, useQuantizedAabbCompression, buildBvh)
         {
         }