Show tile boundaries in debug.html (#858)

tests/debug.html

@@ -285,6 +285,7 @@
     });
 
     map.on('load', async function () {
+        map.showTileBoundaries = true;
         const catalog = await fetch('http://0.0.0.0:3000/catalog');
         const sources = (await catalog.json()).tiles;
         // Set up the corresponding toggle button for each layer.