added enums for fill-rule and vector-effect, and setters in svg.Element.
add a PaintServer abstraction and modify fill and stroke setters to use it.
Remove spaces from rgb serialization. Update tests.
add svg.hex3 convenience function.
svg.Element: add stroke- and fill-related setters and update comments.
Linear+radial gradients: implement PaintServer. Inject id in ctor, update tests.
explain StrokeLineCap and StrokeLineJoin enum values.
<svg>: add setters for viewport-fill and viewport-fill-opacity.
add builder class for <solidColor>. Add tests.
improve test coverage for <line>, <linearGradient>, <radialGradient>, <svg>.