LibWeb/CSS: Absolutize GradientStyleValues

This lets us not care about non-absolute Length units when resolving
gradient data, as they'll already have been converted to px.

We can also use Angle::from_style_value() safely on absolutized angles,
which reduces some boilerplate code.

Libraries/LibWeb/CSS/StyleValues/AbstractImageStyleValue.cpp

@@ -16,6 +16,24 @@ GC::Ref<CSSStyleValue> AbstractImageStyleValue::reify(JS::Realm& realm, FlyStrin
     return CSSImageValue::create(realm, *this);
 }
 
+ColorStopListElement ColorStopListElement::absolutized(ComputationContext const& context) const
+{
+    auto absolutize_if_nonnull = [&context](RefPtr<StyleValue const> const& input) -> RefPtr<StyleValue const> {
+        if (!input)
+            return {};
+        return input->absolutized(context);
+    };
+
+    return {
+        .transition_hint = transition_hint,
+        .color_stop = {
+            .color = absolutize_if_nonnull(color_stop.color),
+            .position = absolutize_if_nonnull(color_stop.position),
+            .second_position = absolutize_if_nonnull(color_stop.second_position),
+        },
+    };
+}
+
 void serialize_color_stop_list(StringBuilder& builder, Vector<ColorStopListElement> const& color_stop_list, SerializationMode mode)
 {
     bool first = true;