Fix filter box dimensions for shadows svg export

Fix filter box dimension for shadows svg exports.
There is one example failing here:
http://fabricjs.com/test/svg_export/bg_patterns_gradients.html

![image](https://cloud.githubusercontent.com/assets/1194048/4783738/6039e0fe-5d34-11e4-9f80-ca375f1938cc.png)

Is there since long ago. I thought it was just the SVG finishing before the shadow but instead the filterbox was too small on X.
By default if not specified the width and height are 120% that is why i added  that + 20, just to have plenty of space.
Why 20? Can we name and/or document it?