Make it possible to custom-paint without an Element.

sky/engine/core/rendering/RenderBox.h

Index: sky/engine/core/rendering/RenderBox.h
diff --git a/sky/engine/core/rendering/RenderBox.h b/sky/engine/core/rendering/RenderBox.h
index bbf8283a8eabb5436b7a526fb5b3cc782ffaaf1f..cbb0194cf956c5589fd1c277c9a081462df70095 100644
--- a/sky/engine/core/rendering/RenderBox.h
+++ b/sky/engine/core/rendering/RenderBox.h
@@ -491,6 +491,8 @@ protected:
 
     void updateIntrinsicContentLogicalHeight(LayoutUnit intrinsicContentLogicalHeight) const { m_intrinsicContentLogicalHeight = intrinsicContentLogicalHeight; }
 
+    void paintCustomPainting(PaintInfo& paintInfo, const LayoutPoint& paintOffset);
+
 private:
     void updateTransformationMatrix();
     void updateTransform(const RenderStyle* oldStyle);