css: fix playlist scrolling performance in chrome

In chrome (and probably other webkit/blink-based browsers) the scrolling
performance of the playlist is awful when there are a lot of items in
the playlist and the individual item buttons are displayed. This change
forces the playlist to be on its own compositing layer, improving
performance significantly.

www/css/cytube.css

@@ -156,6 +156,7 @@
     list-style: none outside none;
     max-height: 500px;
     overflow-y: auto;
+    transform: translateZ(0);
 }