Avoid extra copy during GLTF material parse

indra/newview/llgltfmateriallist.cpp

@@ -333,8 +333,7 @@ namespace
 
 void LLGLTFMaterialList::applyOverrideMessage(LLMessageSystem* msg, const std::string& data_in)
 {
-    std::istringstream str(data_in);
-
+    boost::iostreams::stream<boost::iostreams::array_source> str(data_in.data(), data_in.size());
     LLSD data;
 
     LLSDSerialize::fromNotation(data, str, data_in.length());