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/InProcessWebView.cpp

@@ -110,6 +110,7 @@ void InProcessWebView::select_all()
         last_layout_node_index_in_node = downcast<LayoutText>(*last_layout_node).text_for_rendering().length() - 1;
 
     layout_root->selection().set({ first_layout_node, 0 }, { last_layout_node, last_layout_node_index_in_node });
+    layout_root->recompute_selection_states();
     update();
 }