LibWeb: Add PercentageOr<Length>::to_px() fast path for absolute lengths

We can avoid round-tripping through a temporary Length in the simple
case here.

Userland/Libraries/LibWeb/CSS/PercentageOr.h

@@ -97,6 +97,12 @@ public:
 
     CSSPixels to_px(Layout::Node const& layout_node, CSSPixels reference_value) const
     {
+        if constexpr (IsSame<T, Length>) {
+            if (auto const* length = m_value.template get_pointer<Length>()) {
+                if (length->is_absolute())
+                    return length->absolute_length_to_px();
+            }
+        }
         return resolved(layout_node, reference_value).to_px(layout_node);
     }