Add ability to specify a material assignment to a mesh asset.

This extends both the spec and the XML definition to allow specification of a material name on a mesh. This material acts as a fallback in the case that the referencing geom element does not specify a material of it's own.

PiperOrigin-RevId: 798232378
Change-Id: Iaf300c727732744ea1b613e64334ee505ac6e209

src/user/user_objects.cc

@@ -3365,7 +3365,12 @@ void mjCGeom::ComputeAABB(void) {
   mjuu_copyvec(aabb+3, size, 3);
 }
 
-
+const std::string& mjCGeom::get_material() const {
+  if (mesh && spec_material_.empty()) {
+    return mesh->Material();
+  }
+  return spec_material_;
+}
 
 // compiler
 void mjCGeom::Compile(void) {