Optimise IE filter usage

Added a check around line 1550 to only apply DXImageTransform.Microsoft.Matrix filter if it is needed (if an element is either rotated or scaled).
This is a relevant improvement for two reasons:
* Improved performance in old IE versions: I went up to 25fps from about 15 fps by getting rid of all the Matrix filters on elements that aren't rotated or scaled.
* PNG alpha channel works in IE8: It seems like IE8 can't handle the Matrix filter and display alpha channels properly (see for example: http://stackoverflow.com/questions/7344829/rotating-a-div-with-png-background-using-the-filtermatrix-method-in-ie-turns-tr. All the proposed solutions there actually do not work). Alpha still breaks of course if you rotate or scale an element, haven't found a solution for that yet.