Fix performance issue in applyMoveTransition (#6)

getBoundingClientRect causes a DOM reflow when the DOM has been written to
between calls (see http://stackoverflow.com/a/37686736/945863). So call gBCR
on each element first in one loop with no intermediate writes, then use those
results while writing to the DOM in a second loop.

Author: crabmusket

vue-animated-list.js

@@ -73,6 +73,9 @@
      */
 
     function applyMoveTransition (frags) {
+      frags.forEach(function (frag) {
+        frag._newPos = frag.node.getBoundingClientRect()
+      })
       frags.forEach(function (frag) {
         var node = frag.node
         var oldPos = frag._oldPos
@@ -87,9 +90,8 @@
           p.removeChild(node)
           p.insertBefore(node, next)
         }
-        var newPos = node.getBoundingClientRect()
-        var dx = oldPos.left - newPos.left
-        var dy = oldPos.top - newPos.top
+        var dx = oldPos.left - frag._newPos.left
+        var dy = oldPos.top - frag._newPos.top
         if (dx !== 0 || dy !== 0) {
           frag.moved = true
           node.style.transform =