LibWeb: Add {before,after}_children_paint() methods

This allows layout nodes to do some setup before their children paint,
and cleanup after their children paint. This will be used for SVG
components, where their attributes (like stroke width, fill color, etc)
need to be correctly propogated to layout nodes down the line.

Libraries/LibWeb/Layout/LayoutNode.cpp

@@ -95,11 +95,15 @@ void LayoutNode::paint(PaintContext& context, PaintPhase phase)
     if (!is_visible())
         return;
 
+    before_children_paint(context, phase);
+
     for_each_child([&](auto& child) {
         if (child.is_box() && downcast<LayoutBox>(child).stacking_context())
             return;
         child.paint(context, phase);
     });
+
+    after_children_paint(context, phase);
 }
 
 HitTestResult LayoutNode::hit_test(const Gfx::IntPoint& position, HitTestType type) const