LibWeb: Optimize :hover style invalidation

Instead of recalculating styles for all nodes in the common ancestor of
the new and old hovered nodes' subtrees, this change introduces the
following approach:
- While calculating ComputedProperties, a flag is saved if any rule
  applied to an element is affected by the hover state during the
  execution of SelectorEngine::matches().
- When the hovered element changes, styles are marked for recalculation
  only if the flag saved in ComputedProperties indicates that the
  element could be affected by the hover state.

Libraries/LibWeb/CSS/ComputedProperties.h

@@ -213,6 +213,9 @@ public:
 
     static float resolve_opacity_value(CSSStyleValue const& value);
 
+    bool did_match_any_hover_rules() const { return m_did_match_any_hover_rules; }
+    void set_did_match_any_hover_rules() { m_did_match_any_hover_rules = true; }
+
 private:
     friend class StyleComputer;
 
@@ -236,6 +239,8 @@ private:
     mutable RefPtr<Gfx::FontCascadeList> m_font_list;
 
     Optional<CSSPixels> m_line_height;
+
+    bool m_did_match_any_hover_rules { false };
 };
 
 }