Move meshdir and texturedir to mjsCompiler.

For backwards compatibility in the Python bindings, both `spec.compiler.meshdir` and `spec.meshdir` are now allowed.

Fixes #2834.

PiperOrigin-RevId: 809060113
Change-Id: Ie23f1c5dd57de43568312a8e062906f4013bb474

src/user/user_api.cc

@@ -297,7 +297,7 @@ int mjs_isWarning(mjSpec* s) {
 void mj_deleteSpec(mjSpec* s) {
   if (s) {
     mjCModel* model = static_cast<mjCModel*>(s->element);
-    delete model;
+    model->Release();
   }
 }