Fix export of deleted meshes; Add LazyDict::Remove method

When I was merging a node’s multiple meshes into that node’s first mesh’s primitives, I was deleting the merged meshes from the node.

However, I wasn’t deleting the merged meshes from the mAsset->meshes Dict, causing the gltf2 export to contain extra unreferenced meshes and duplicate primitives.

This new code adds a new method to LazyDict, which removes the object from it, taking care to update indexes of the subsequent objects. This change also requires that `Ref`s of `Mesh`es (stored in node->meshes) have their indexes updated.

code/glTF2Exporter.cpp

@@ -800,9 +800,33 @@ void glTF2Exporter::MergeMeshes()
             for (unsigned int m = nMeshes - 1; m >= 1; --m) {
                 Ref<Mesh> mesh = node->meshes.at(m);
 
-                firstMesh->primitives.insert(firstMesh->primitives.end(), mesh->primitives.begin(), mesh->primitives.end());
+                //append this mesh's primitives to the first mesh's primitives
+                firstMesh->primitives.insert(
+                    firstMesh->primitives.end(),
+                    mesh->primitives.begin(),
+                    mesh->primitives.end()
+                );
 
-                node->meshes.erase(node->meshes.begin() + m);
+                //remove the mesh from the list of meshes
+                unsigned int removedIndex = mAsset->meshes.Remove(mesh->id.c_str());
+
+                //find the presence of the removed mesh in other nodes
+                for (unsigned int nn = 0; nn < mAsset->nodes.Size(); ++nn) {
+                    Ref<Node> node = mAsset->nodes.Get(nn);
+
+                    for (unsigned int mm = 0; mm < node->meshes.size(); ++mm) {
+                        Ref<Mesh>& meshRef = node->meshes.at(mm);
+                        unsigned int meshIndex = meshRef.GetIndex();
+
+                        if (meshIndex == removedIndex) {
+                            node->meshes.erase(node->meshes.begin() + mm);
+                        } else if (meshIndex > removedIndex) {
+                            Ref<Mesh> newMeshRef = mAsset->meshes.Get(meshIndex - 1);
+
+                            meshRef = newMeshRef;
+                        }
+                    }
+                }
             }
 
             //since we were looping backwards, reverse the order of merged primitives to their original order