3mf: use correct material assignment in case of multi-materials.

code/D3MFExporter.cpp

@@ -126,7 +126,6 @@ bool D3MFExporter::exportArchive( const char *file ) {
     return ok;
 }
 
-
 bool D3MFExporter::exportContentTypes() {
     mContentOutput.clear();
 
@@ -177,6 +176,8 @@ bool D3MFExporter::export3DModel() {
     mModelOutput << "<" << XmlTag::resources << ">";
     mModelOutput << std::endl;
 
+    writeBaseMaterials();
+
     writeObjects();
 
 
@@ -203,6 +204,10 @@ void D3MFExporter::writeHeader() {
     mModelOutput << std::endl;
 }
 
+void D3MFExporter::writeBaseMaterials() {
+
+}
+
 void D3MFExporter::writeObjects() {
     if ( nullptr == mScene->mRootNode ) {
         return;