🐎 only calculate .scrollTop() once in scroll callback

Author: Daniel Ma

parallax.js

@@ -272,11 +272,12 @@
           Parallax.requestRender();
         })
         .on('scroll.px.parallax load.px.parallax', function() {
+          var winScrollTop  = $win.scrollTop();
           var scrollTopMax  = Parallax.docHeight - Parallax.winHeight;
           var scrollLeftMax = Parallax.docWidth  - Parallax.winWidth;
-          Parallax.scrollTop  = Math.max(0, Math.min(scrollTopMax,  $win.scrollTop()));
+          Parallax.scrollTop  = Math.max(0, Math.min(scrollTopMax,  winScrollTop));
           Parallax.scrollLeft = Math.max(0, Math.min(scrollLeftMax, $win.scrollLeft()));
-          Parallax.overScroll = Math.max($win.scrollTop() - scrollTopMax, Math.min($win.scrollTop(), 0));
+          Parallax.overScroll = Math.max(winScrollTop - scrollTopMax, Math.min(winScrollTop, 0));
           Parallax.requestRender();
         });