Replace mjCModel with mjmModel in mjXBase.

PiperOrigin-RevId: 611762880
Change-Id: I22254689b6cbf10fe7b61f9742b6ffd6c2074477

src/xml/xml_native_writer.cc

@@ -764,6 +764,14 @@ mjXWriter::mjXWriter(void) {
 }
 
 
+// cast model
+void mjXWriter::SetModel(mjmModel* modelspec) {
+  if (modelspec) {
+    model = (mjCModel*)modelspec->element;
+  }
+}
+
+
 // save existing model in MJCF canonical format, must be compiled
 string mjXWriter::Write(char *error, size_t error_sz) {
   // check model