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

@@ -28,7 +28,7 @@
 #include <simgear/compiler.h>
 
 #include <map>
-
+#include <mutex>
 
 #include <osg/AlphaFunc>
 #include <osg/Group>
@@ -103,6 +103,8 @@ 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++) {

simgear/scene/material/matmodel.hxx

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