Helios - LYN-3250 - Fixed morph targets for meshes that had multiple … (#696)

* Helios - LYN-3250 - Fixed morph targets for meshes that had multiple materials (#374)

Fixed morph targets for meshes that had multiple materials and were split by AssImp: Recombined them into one mesh in the O3DE scene graph, so the behavior would match FBX SDK.

Code/Tools/SceneAPI/SDKWrapper/AssImpSceneWrapper.cpp

@@ -38,12 +38,14 @@ namespace AZ
         {
             AZ_TracePrintf(SceneAPI::Utilities::LogWindow, "AssImpSceneWrapper::LoadSceneFromFile %s", fileName);
             AZ_TraceContext("Filename", fileName);
+            // aiProcess_JoinIdenticalVertices is not enabled because O3DE has a mesh optimizer that also does this,
+            // this flag is disabled to keep AssImp output similar to FBX SDK to reduce downstream bugs for the initial AssImp release.
+            // There's currently a minimum of properties and flags set to maximize compatibility with the existing node graph.
             m_importer.SetPropertyBool(AI_CONFIG_IMPORT_FBX_PRESERVE_PIVOTS, false);
             m_importer.SetPropertyBool(AI_CONFIG_IMPORT_FBX_OPTIMIZE_EMPTY_ANIMATION_CURVES, false);
             m_sceneFileName = fileName;
             m_assImpScene = m_importer.ReadFile(fileName,
                 aiProcess_Triangulate //Triangulates all faces of all meshes
-                | aiProcess_JoinIdenticalVertices //Identifies and joins identical vertex data sets for the imported meshes
                 | aiProcess_LimitBoneWeights //Limits the number of bones that can affect a vertex to a maximum value
                                              //dropping the least important and re-normalizing
                 | aiProcess_GenNormals); //Generate normals for meshes