Bugfix: incorrect animation endpoints in some cases of parent resize

Bug exists in following conditions:
- parent container is centered (css: "margin:auto")
- browser shows or hides scrollbar after/during elements filtering

In such case, parent container moves few pixels to the left or right, but filtered elements uses its old position to calculate animation endpoints. That results in blocks hopping after or before the animation.

The scrollbar may not even be visible to the user - in the following example, it's "visible" only during (very short) intermediate state of position calculations.

Example:
https://dl.dropboxusercontent.com/u/5448886/contrib/mixitup/bugexample1.html

Tested on Firefox 24, Opera 12, Chromium 26 (all on Linux).