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.cpp

@@ -0,0 +1,78 @@
+/*
+ * Copyright (c) 2023, MacDue <macdue@dueutil.tech>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/DOM/Document.h>
+#include <LibWeb/SVG/AttributeNames.h>
+#include <LibWeb/SVG/SVGGradientElement.h>
+#include <LibWeb/SVG/SVGGraphicsElement.h>
+
+namespace Web::SVG {
+
+SVGGradientElement::SVGGradientElement(DOM::Document& document, DOM::QualifiedName qualified_name)
+    : SVGElement(document, move(qualified_name))
+{
+}
+
+void SVGGradientElement::parse_attribute(DeprecatedFlyString const& name, DeprecatedString const& value)
+{
+    SVGElement::parse_attribute(name, value);
+    if (name == AttributeNames::gradientUnits) {
+        m_gradient_units = AttributeParser::parse_gradient_units(value);
+    } else if (name == AttributeNames::gradientTransform) {
+        if (auto transform_list = AttributeParser::parse_transform(value); transform_list.has_value()) {
+            m_gradient_transform = transform_from_transform_list(*transform_list);
+        } else {
+            m_gradient_transform = {};
+        }
+    }
+}
+
+GradientUnits SVGGradientElement::gradient_units() const
+{
+    if (m_gradient_units.has_value())
+        return *m_gradient_units;
+    if (auto href = xlink_href())
+        return href->gradient_units();
+    return GradientUnits::ObjectBoundingBox;
+}
+
+Optional<Gfx::AffineTransform> SVGGradientElement::gradient_transform() const
+{
+    if (m_gradient_transform.has_value())
+        return m_gradient_transform;
+    if (auto href = xlink_href())
+        return href->gradient_transform();
+    return {};
+}
+
+JS::GCPtr<SVGGradientElement const> SVGGradientElement::xlink_href() const
+{
+    // FIXME: This entire function is an ad-hoc hack!
+    // It can only resolve #<ids> in the same document.
+    if (auto href = get_attribute("href"); !href.is_empty()) {
+        auto url = document().parse_url(href);
+        auto id = url.fragment();
+        if (id.is_empty())
+            return {};
+        auto element = document().get_element_by_id(id);
+        if (!element)
+            return {};
+        if (!is<SVGGradientElement>(*element))
+            return {};
+        return &verify_cast<SVGGradientElement>(*element);
+    }
+    return {};
+}
+
+JS::ThrowCompletionOr<void> SVGGradientElement::initialize(JS::Realm& realm)
+{
+    MUST_OR_THROW_OOM(Base::initialize(realm));
+    set_prototype(&Bindings::ensure_web_prototype<Bindings::SVGGradientElementPrototype>(realm, "SVGGradientElement"));
+    return {};
+}
+
+}