Fix studio web so that single file xml model can be loaded in a single-threaded wasm build

model_holder.cc modified to disable multi-threaded compilation for WASM builds. index.html `animate()` function modified so the animation rendering loop executes asynchronously with async/await calls. These changes are sufficient to make xml files without references work (e.g., humanoid.xml), more work is needed to make the vfs work so multi-file models can load.

PiperOrigin-RevId: 885537402
Change-Id: I3469cae48e7268073be334b99bf907c5ed8c27a7

src/experimental/platform/model_holder.cc

@@ -80,6 +80,10 @@ ModelHolder::~ModelHolder() {
 
 void ModelHolder::PostInit() {
   if (spec_ && !model_) {
+#ifdef __EMSCRIPTEN__
+    // Disable threaded compilation on WASM since pthreads are not available.
+    spec_->compiler.usethread = 0;
+#endif
     model_ = mj_compile(spec_, &vfs_);
     if (!model_) {
       SetLoadError(mjs_getError(spec_));

src/experimental/studio/index.html

@@ -97,9 +97,9 @@
               console.error('Failed to prefetch one or more assets.', error);
             });
         },
-        animate: () => {
+        animate: async () => {
           try {
-            Module.renderFrame();
+            await Module.renderFrame();
           } catch (error) {
             console.error('Update error:', error);
           }