LibWeb/CSS: Implement CursorStyleValue

A cursor is an image, with an optional x,y hotspot.

We know that a CursorStyleValue's bitmap never needs to change size, so
we create the ShareableBitmap once and then cache it, so that we don't
have to repeatedly create an FD for it or do the work of painting.

To avoid repainting that bitmap, we cache the values that were used to
create it - what currentColor is and its length resolution context -
and only repaint when those change.

Libraries/LibWeb/CSS/StyleValues/CursorStyleValue.h

@@ -0,0 +1,64 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Optional.h>
+#include <LibGfx/Color.h>
+#include <LibGfx/Cursor.h>
+#include <LibWeb/CSS/CSSStyleValue.h>
+#include <LibWeb/CSS/CalculatedOr.h>
+#include <LibWeb/CSS/Length.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::CSS {
+
+class CursorStyleValue final : public StyleValueWithDefaultOperators<CursorStyleValue> {
+public:
+    static ValueComparingNonnullRefPtr<CursorStyleValue> create(ValueComparingNonnullRefPtr<AbstractImageStyleValue> image, Optional<NumberOrCalculated> x, Optional<NumberOrCalculated> y)
+    {
+        VERIFY(x.has_value() == y.has_value());
+        return adopt_ref(*new (nothrow) CursorStyleValue(move(image), move(x), move(y)));
+    }
+    virtual ~CursorStyleValue() override = default;
+
+    ValueComparingNonnullRefPtr<AbstractImageStyleValue> image() const { return m_properties.image; }
+    Optional<NumberOrCalculated> const& x() const { return m_properties.x; }
+    Optional<NumberOrCalculated> const& y() const { return m_properties.y; }
+
+    Optional<Gfx::ImageCursor> make_image_cursor(Layout::NodeWithStyle const&) const;
+
+    virtual String to_string(SerializationMode) const override;
+
+    bool properties_equal(CursorStyleValue const& other) const { return m_properties == other.m_properties; }
+
+private:
+    CursorStyleValue(ValueComparingNonnullRefPtr<AbstractImageStyleValue> image,
+        Optional<NumberOrCalculated> x,
+        Optional<NumberOrCalculated> y)
+        : StyleValueWithDefaultOperators(Type::Cursor)
+        , m_properties { .image = move(image), .x = move(x), .y = move(y) }
+    {
+    }
+
+    struct Properties {
+        ValueComparingNonnullRefPtr<AbstractImageStyleValue> image;
+        Optional<NumberOrCalculated> x;
+        Optional<NumberOrCalculated> y;
+        bool operator==(Properties const&) const = default;
+    } m_properties;
+
+    // Data that can affect the bitmap rendering.
+    struct CacheKey {
+        Length::ResolutionContext length_resolution_context;
+        Gfx::Color current_color;
+        bool operator==(CacheKey const&) const = default;
+    };
+    mutable Optional<CacheKey> m_cache_key;
+    mutable Optional<Gfx::ShareableBitmap> m_cached_bitmap;
+};
+
+}