LibWeb: Implement SVGGradientElement

This is the base class for all SVG gradient types. This supports:

- The `gradientUnits` attribute
- The `gradientTransform` attribute
- And following `xlink:hrefs` for inheriting <stops>/attributes

Userland/Libraries/LibWeb/SVG/SVGGradientElement.h

@@ -0,0 +1,59 @@
+/*
+ * Copyright (c) 2023, MacDue <macdue@dueutil.tech>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/IterationDecision.h>
+#include <LibGfx/PaintStyle.h>
+#include <LibWeb/SVG/AttributeParser.h>
+#include <LibWeb/SVG/SVGElement.h>
+#include <LibWeb/SVG/SVGStopElement.h>
+
+namespace Web::SVG {
+
+struct SVGPaintContext {
+    Gfx::FloatRect viewport;
+    Gfx::FloatRect path_bounding_box;
+    Gfx::AffineTransform transform;
+};
+
+class SVGGradientElement : public SVGElement {
+    WEB_PLATFORM_OBJECT(SVGGradientElement, SVGElement);
+
+public:
+    virtual ~SVGGradientElement() override = default;
+
+    virtual void parse_attribute(DeprecatedFlyString const& name, DeprecatedString const& value) override;
+
+    virtual Optional<Gfx::PaintStyle const&> to_gfx_paint_style(SVGPaintContext const&) const = 0;
+
+    GradientUnits gradient_units() const;
+
+    Optional<Gfx::AffineTransform> gradient_transform() const;
+
+protected:
+    SVGGradientElement(DOM::Document&, DOM::QualifiedName);
+
+    virtual JS::ThrowCompletionOr<void> initialize(JS::Realm&) override;
+
+    JS::GCPtr<SVGGradientElement const> xlink_href() const;
+
+    template<VoidFunction<SVGStopElement> Callback>
+    void for_each_color_stop(Callback const& callback) const
+    {
+        for_each_child_of_type<SVG::SVGStopElement>([&](auto& stop) {
+            callback(stop);
+        });
+        if (auto href = xlink_href())
+            href->for_each_color_stop(callback);
+    }
+
+private:
+    Optional<GradientUnits> m_gradient_units = {};
+    Optional<Gfx::AffineTransform> m_gradient_transform = {};
+};
+
+}