Update fade to use elapsed time to fix inconsistencies

Fixes goldfire#885

src/howler.core.js

@@ -1166,28 +1166,20 @@
     _startFadeInterval: function(sound, from, to, len, id, isGroup) {
       var self = this;
       var vol = from;
-      var dir = from > to ? 'out' : 'in';
-      var diff = Math.abs(from - to);
-      var steps = diff / 0.01;
-      var stepLen = (steps > 0) ? len / steps : len;
-      var stepVol = diff / steps;
-
-      // Since browsers clamp timeouts to 4ms, we need to clamp our steps to that too.
-      if (stepLen < 4) {
-        steps = Math.ceil(steps * stepLen * 0.25);
-        stepLen = 4;
-        stepVol = diff / steps;
-      }
+      var diff = to - from;
+      var steps = Math.abs(diff / 0.01);
+      var stepLen = Math.max(4, (steps > 0) ? len / steps : len);
+      var lastTick = Date.now();
 
       // Store the value being faded to.
       sound._fadeTo = to;
 
       // Update the volume value on each interval tick.
       sound._interval = setInterval(function() {
-        // Update the volume amount, but only if the volume should change.
-        if (steps > 0) {
-          vol += (dir === 'in' ? stepVol : -stepVol);
-        }
+        // Update the volume based on the time since the last tick.
+        var tick = (Date.now() - lastTick) / len;
+        lastTick = Date.now();
+        vol += diff * tick;
 
         // Make sure the volume is in the right bounds.
         vol = Math.max(0, vol);