Gradient fix

Added rendering of gradientTransform, fixed gradient position ( not working since big commit of svg import ) fixed use of xlink:href in gradients, stop opacity 0% was omitted. Fixed toSVG for gradients.
Added gradients for stroke property.

Related to issue #1597
Let's call `postProcess` something more clear, since it's difficult to figure out what exactly is in it. Is it kind of like `idsToUrlsMap`?