Refactor model loading APIs.

There are three ways to load a model: from a file, from a buffer, or an
"empty" model. Instead of overloading a single function to do all three,
provide three separate functions.

Also disable model loading functions that will not work for emscripten.

PiperOrigin-RevId: 859051341
Change-Id: I71557d31d0e7c9c6882895c36116d300f19e255d

src/experimental/studio/main.cc

@@ -105,7 +105,13 @@ int main(int argc, char** argv, char** envp) {
   // If the model file is not specified, try to load it from the first argument
   std::string model_file = absl::GetFlag(FLAGS_model_file);
   if (model_file.empty() && argc > 1 && argv[1][0] != '-') model_file = argv[1];
-  app.LoadModel(model_file, mujoco::studio::App::ContentType::kFilepath);
+
+  if (model_file.empty()) {
+    app.InitEmptyModel();
+  } else {
+    app.LoadModelFromFile(model_file);
+  }
+
   while (app.Update()) {
     app.BuildGui();
     app.Render();