LibWeb: Add an initial implementation of SVG <radialGradient>

This follows on from the SVG linear gradients. It supports the same
features (xlink:href, gradientUnits, gradientTransform).

With this commit I have now implemented all web gradients :^)

(Though we are still missing a few parameters for SVG gradients,
e.g. spreadMethod).

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -510,6 +510,7 @@ set(SOURCES
     SVG/SVGPolygonElement.cpp
     SVG/SVGPolylineElement.cpp
     SVG/SVGRectElement.cpp
+    SVG/SVGRadialGradientElement.cpp
     SVG/SVGSVGElement.cpp
     SVG/SVGStopElement.cpp
     SVG/SVGTextContentElement.cpp