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/xml/xml_native_reader.cc

@@ -230,9 +230,9 @@ const char* MJCF[nMJCF][mjXATTRNUM] = {
 
     {"asset", "*", "0"},
     {"<"},
-        {"mesh", "*", "16", "name", "class", "content_type", "file", "vertex", "normal",
+        {"mesh", "*", "17", "name", "class", "content_type", "file", "vertex", "normal",
             "texcoord", "face", "refpos", "refquat", "scale", "smoothnormal",
-            "maxhullvert", "inertia", "builtin", "params"},
+            "maxhullvert", "inertia", "builtin", "params", "material"},
         {"<"},
           {"plugin", "*", "2", "plugin", "instance"},
           {"<"},
@@ -1569,6 +1569,11 @@ void mjXReader::OneMesh(XMLElement* elem, mjsMesh* mesh, const mjVFS* vfs) {
     }
   }
 
+  std::string material;
+  if (ReadAttrTxt(elem, "material", material)) {
+    mjs_setString(mesh->material, material.c_str());
+  }
+
   // write error info
   mjs_setString(mesh->info, ("line " + std::to_string(elem->GetLineNum())).c_str());
 }