LibWeb: Add GradientPainting for painting linear-gradients

This implements support for painting linear-gradients in a spec
correct way :^).

Right now it supports:

- Multi-stop gradients
- Color stop fixups
- Using pre-multiplied alpha mixing when required
- Painting gradients at arbitrary angles

It still needs to support:

- Transition hints
- Double position color stops

However what is implemented now seems to be accurate to other
browsers, and covers the most common use cases.

Userland/Libraries/LibWeb/Painting/GradientPainting.h

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2022, MacDue <macdue@dueutil.tech>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Span.h>
+#include <AK/Vector.h>
+#include <LibGfx/Color.h>
+#include <LibWeb/CSS/StyleValue.h>
+#include <LibWeb/Forward.h>
+#include <LibWeb/Painting/PaintContext.h>
+
+namespace Web::Painting {
+
+struct ColorStop {
+    Gfx::Color color;
+    float position = 0;
+};
+
+using ColorStopList = Vector<ColorStop, 4>;
+
+struct LinearGradientData {
+    float gradient_angle;
+    ColorStopList color_stops;
+};
+
+LinearGradientData resolve_linear_gradient_data(Layout::Node const&, Gfx::FloatRect const&, CSS::LinearGradientStyleValue const&);
+
+void paint_linear_gradient(PaintContext&, Gfx::IntRect const&, LinearGradientData const&);
+
+}