LibWeb: Introduce color-function-specific style values

Instead of CSSColorValue holding a Gfx::Color, make it an abstract class
with subclasses for each different color function, to match the Typed-OM
spec. This means moving the color calculations from the parsing code to
the `to_color()` method on the style value.

This lets us have calc() inside a color function, instead of having to
fully resolve the color at parse time. The canvas fillStyle tests have
been updated to reflect this.

The other test change is Screenshot/css-color-functions.html: previously
we produced slightly different colors for an alpha of 0.5 and one of
50%, and this incorrect behavior was baked into the test. So now it's
more correct. :^)

Userland/Libraries/LibWeb/CSS/StyleValues/CSSRGB.cpp

@@ -0,0 +1,77 @@
+/*
+ * Copyright (c) 2024, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "CSSRGB.h"
+#include <AK/TypeCasts.h>
+#include <LibWeb/CSS/Serialize.h>
+#include <LibWeb/CSS/StyleValues/CalculatedStyleValue.h>
+#include <LibWeb/CSS/StyleValues/NumberStyleValue.h>
+#include <LibWeb/CSS/StyleValues/PercentageStyleValue.h>
+
+namespace Web::CSS {
+
+Color CSSRGB::to_color(Optional<Layout::NodeWithStyle const&>) const
+{
+    auto resolve_rgb_to_u8 = [](CSSStyleValue const& style_value) -> Optional<u8> {
+        // <number> | <percentage> | none
+        auto normalized = [](double number) {
+            return llround(clamp(number, 0.0, 255.0));
+        };
+
+        if (style_value.is_number())
+            return normalized(style_value.as_number().number());
+
+        if (style_value.is_percentage())
+            return normalized(style_value.as_percentage().value() * 2.55);
+
+        if (style_value.is_calculated()) {
+            auto const& calculated = style_value.as_calculated();
+            if (calculated.resolves_to_number())
+                return normalized(calculated.resolve_number().value());
+            if (calculated.resolves_to_percentage())
+                return normalized(calculated.resolve_percentage().value().value() * 2.55);
+        }
+
+        if (style_value.is_keyword() && style_value.to_keyword() == Keyword::None)
+            return 0;
+
+        return {};
+    };
+
+    auto resolve_alpha_to_u8 = [](CSSStyleValue const& style_value) -> Optional<u8> {
+        auto alpha_0_1 = resolve_alpha(style_value);
+        if (alpha_0_1.has_value())
+            return llround(clamp(alpha_0_1.value() * 255.0, 0.0, 255.0));
+        return {};
+    };
+
+    u8 const r_val = resolve_rgb_to_u8(m_properties.r).value_or(0);
+    u8 const g_val = resolve_rgb_to_u8(m_properties.g).value_or(0);
+    u8 const b_val = resolve_rgb_to_u8(m_properties.b).value_or(0);
+    u8 const alpha_val = resolve_alpha_to_u8(m_properties.alpha).value_or(255);
+
+    return Color(r_val, g_val, b_val, alpha_val);
+}
+
+bool CSSRGB::equals(CSSStyleValue const& other) const
+{
+    if (type() != other.type())
+        return false;
+    auto const& other_color = other.as_color();
+    if (color_type() != other_color.color_type())
+        return false;
+    auto const& other_rgb = verify_cast<CSSRGB>(other_color);
+    return m_properties == other_rgb.m_properties;
+}
+
+// https://www.w3.org/TR/css-color-4/#serializing-sRGB-values
+String CSSRGB::to_string() const
+{
+    // FIXME: Do this properly, taking unresolved calculated values into account.
+    return serialize_a_srgb_value(to_color({}));
+}
+
+}