LibWeb: Add barebones CanvasGradient object

Also add the CanvasRenderingContext2D APIs to go along with it.
Note that it can't be used for anything yet.

Userland/Libraries/LibWeb/HTML/CanvasGradient.h

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/RefCounted.h>
+#include <LibWeb/Bindings/Wrappable.h>
+
+namespace Web::HTML {
+
+class CanvasGradient final
+    : public RefCounted<CanvasGradient>
+    , public Bindings::Wrappable {
+public:
+    using WrapperType = Bindings::CanvasGradientWrapper;
+
+    enum class Type {
+        Linear,
+        Radial,
+        Conic,
+    };
+
+    static NonnullRefPtr<CanvasGradient> create_radial(double x0, double y0, double r0, double x1, double y1, double r1);
+    static NonnullRefPtr<CanvasGradient> create_linear(double x0, double y0, double x1, double y1);
+    static NonnullRefPtr<CanvasGradient> create_conic(double start_angle, double x, double y);
+
+    void add_color_stop(double offset, String const& color);
+
+    ~CanvasGradient();
+
+private:
+    explicit CanvasGradient(Type);
+
+    Type m_type {};
+};
+
+}