Avoid a race in MatModels loading

Add a mutex to ensure an SGMatModels only loads its models once. Caught
be ASan, hurrah.

simgear/scene/material/matmodel.cxx

@@ -103,10 +103,12 @@ SGMatModel::get_model_count( SGPropertyNode *prop_root )
 inline void
 SGMatModel::load_models( SGPropertyNode *prop_root )
 {
+    std::lock_guard<std::mutex> g(_loadMutex);
+
     // Load model only on demand
     if (!_models_loaded) {
         for (unsigned int i = 0; i < _paths.size(); i++) {
-      osg::Node *entity = SGModelLib::loadModel(_paths[i], prop_root);
+            osg::Node* entity = SGModelLib::loadModel(_paths[i], prop_root);
             if (entity != 0) {
                 // FIXME: this stuff can be handled
                 // in the XML wrapper as well (at least,

simgear/scene/material/matmodel.hxx

@@ -149,6 +149,8 @@ private:
     double _spacing_m;
     double _range_m;
     HeadingType _heading_type;
+
+    std::mutex _loadMutex;
 };