Add material namespacing to sites.

Fixes #2243.

PiperOrigin-RevId: 699713625
Change-Id: I1b45e43f2a6ccfa64c46489c9426fcc2acdc94c3

src/user/user_objects.cc

@@ -3102,6 +3102,15 @@ void mjCSite::CopyFromSpec() {
 
 
 
+void mjCSite::NameSpace(const mjCModel* m) {
+  mjCBase::NameSpace(m);
+  if (!spec_material_.empty() && model != m) {
+    spec_material_ = m->prefix + spec_material_ + m->suffix;
+  }
+}
+
+
+
 // compiler
 void mjCSite::Compile(void) {
   CopyFromSpec();