LibWeb: Resolve and paint SVG gradient fills

This bit is mostly ad-hoc for now. This simply turns fill: url(#grad1)
into document().get_element_by_id('grad1') then resolves the gradient.
This seems to do the trick for most use cases, but this is not
attempting to follow the spec yet to keep things simple.

Userland/Libraries/LibWeb/SVG/SVGGraphicsElement.h

@@ -7,10 +7,12 @@
 
 #pragma once
 
+#include <LibGfx/PaintStyle.h>
 #include <LibGfx/Path.h>
 #include <LibWeb/DOM/Node.h>
 #include <LibWeb/SVG/AttributeParser.h>
 #include <LibWeb/SVG/SVGElement.h>
+#include <LibWeb/SVG/SVGGradientElement.h>
 #include <LibWeb/SVG/TagNames.h>
 
 namespace Web::SVG {
@@ -37,6 +39,8 @@ public:
 
     Gfx::AffineTransform get_transform() const;
 
+    Optional<Gfx::PaintStyle const&> fill_paint_style(SVGPaintContext const&) const;
+
 protected:
     SVGGraphicsElement(DOM::Document&, DOM::QualifiedName);