Merge pull request #3839 from Garux/mRootNode_mTransformation

consider pScene->mRootNode->mTransformation set by some importers while using AI_CONFIG_PP_PTV_ROOT_TRANSFORMATION

code/PostProcessing/PretransformVertices.cpp

@@ -429,7 +429,7 @@ void PretransformVertices::Execute(aiScene *pScene) {
 	const unsigned int iOldNodes = CountNodes(pScene->mRootNode);
 
 	if (configTransform) {
-		pScene->mRootNode->mTransformation = configTransformation;
+		pScene->mRootNode->mTransformation = configTransformation * pScene->mRootNode->mTransformation;
 	}
 
 	// first compute absolute transformation matrices for all nodes