gl.js: use performance timer

Use performance.now instead of Date.now, which may provide a
higher-resolution time (depending on the browser's Spectre mitigation
settings). The time returned will no longer be relative to the Unix
epoch and system clock.

js/gl.js

@@ -615,7 +615,7 @@ var importObject = {
             return Math.floor(Math.random() * 2147483647);
         },
         now: function () {
-            return Date.now() / 1000.0;
+            return performance.now() / 1000.0;
         },
         canvas_width: function () {
             return Math.floor(canvas.width);