LibWeb: Remember the selection state of each LayoutNode

Instead of computing it on the fly while painting each layout node,
they now remember their selection state. This avoids a whole bunch
of tree traversal while painting with anything selected.

Libraries/LibWeb/Layout/LayoutDocument.h

@@ -54,6 +54,8 @@ public:
 
     void build_stacking_context_tree();
 
+    void recompute_selection_states();
+
 private:
     LayoutRange m_selection;
 };