Switch vals to thread_locals

It's not possible to share them across threads, so in case we decide to use multithreading in the future, it's best to mark them as thread_local right away, even if it's a no-op right now.

codecs/mozjpeg_enc/example.html

@@ -20,28 +20,29 @@
   module.onRuntimeInitialized = async _ => {
     console.log('Version:', module.version().toString(16));
     const image = await loadImage('../example.png');
-    let results = [];
-    for (let i = 0; i < 2; i++) {
-      results.push(module.encode(image.data, image.width, image.height, {
-        quality: 75,
-        baseline: false,
-        arithmetic: false,
-        progressive: true,
-        optimize_coding: true,
-        smoothing: 0,
-        color_space: 3,
-        quant_table: 3,
-        trellis_multipass: false,
-        trellis_opt_zero: false,
-        trellis_opt_table: false,
-        trellis_loops: 1,
-        auto_subsample: true,
-        chroma_subsample: 2,
-        separate_chroma_quality: false,
-        chroma_quality: 75,
-      }));
-    }
+    const result = module.encode(image.data, image.width, image.height, {
+      quality: 75,
+      baseline: false,
+      arithmetic: false,
+      progressive: true,
+      optimize_coding: true,
+      smoothing: 0,
+      color_space: 3,
+      quant_table: 3,
+      trellis_multipass: false,
+      trellis_opt_zero: false,
+      trellis_opt_table: false,
+      trellis_loops: 1,
+      auto_subsample: true,
+      chroma_subsample: 2,
+      separate_chroma_quality: false,
+      chroma_quality: 75,
+    });
 
-    console.log(results);
+    const blob = new Blob([result], {type: 'image/jpeg'});
+    const blobURL = URL.createObjectURL(blob);
+    const img = document.createElement('img');
+    img.src = blobURL;
+    document.body.appendChild(img);
   };
 </script>