[FIX](protovis gradient multiple) Generally, multiple distinct gradients could render incorrectly, due to sharing of FillStyle/Color objects to store the multiple "color" urls that would refer to gradients defined in distinct parent elements. Urls would point to out-of-scope gradients. Now, all gradients are defined at the svg/defs element and are shared between FillStyles/Colors having the same key. There is no GC of gradients; it is assumed that the number of distinct gradients per root is relatively small, and that these are stable along consecutive renders.