draw-through

it is possible to render with OpenGL/DX by using draw-through.

include/nana/gui/basis.hpp

@@ -25,6 +25,7 @@ namespace nana
 		struct native_window_handle_impl{};
 		struct window_handle_impl{};
 		struct event_handle_impl{};
+		struct native_drawable_impl{};
 	}
 
 	enum class checkstate
@@ -66,6 +67,7 @@ namespace nana
 	using native_window_type = detail::native_window_handle_impl*;
 	using window = detail::window_handle_impl*; ///< \see [What is window class ](https://sourceforge.net/p/nanapro/discussion/general/thread/bd0fabfb/) 
 	using event_handle = detail::event_handle_impl*;
+	using native_drawable_type = detail::native_drawable_impl*;
 
 
 	struct keyboard

include/nana/gui/detail/basic_window.hpp

@@ -74,7 +74,7 @@ namespace detail
 	struct basic_window
 		: public events_holder
 	{
-		typedef std::vector<basic_window*> container;
+		using container = std::vector<basic_window*>;
 
 		struct root_context
 		{
@@ -119,6 +119,8 @@ namespace detail
 		bool is_ancestor_of(const basic_window* wd) const;
 		bool visible_parents() const;
 		bool belong_to_lazy() const;
+
+		bool is_draw_through() const;	///< Determines whether it is a draw-through window.
 	public:
 		//Override event_holder
 		bool set_events(const std::shared_ptr<general_events>&) override;
@@ -206,6 +208,8 @@ namespace detail
 #endif
 				cursor			state_cursor{nana::cursor::arrow};
 				basic_window*	state_cursor_window{ nullptr };
+
+				std::function<void()> draw_through;	///< A draw through renderer for root widgets.
 			};
 
 			const category::flags category;

include/nana/gui/detail/bedrock.hpp

@@ -67,6 +67,7 @@ namespace detail
 		bool whether_keyboard_shortkey() const;
 
 		element_store& get_element_store() const;
+		void map_through_widgets(core_window_t*, native_drawable_type);
 	public:
 		void event_expose(core_window_t *, bool exposed);
 		void event_move(core_window_t*, int x, int y);

include/nana/gui/programming_interface.hpp

@@ -115,7 +115,8 @@ namespace API
 
 	void window_icon_default(const paint::image&);
 	void window_icon(window, const paint::image&);
-	bool empty_window(window);                            ///< Determines whether a window is existing.
+	bool empty_window(window);		///< Determines whether a window is existing.
+	bool is_window(window);			///< Determines whether a window is existing, equal to !empty_window.
 	void enable_dropfiles(window, bool);
 
     /// \brief Retrieves the native window of a Nana.GUI window.
@@ -176,6 +177,9 @@ namespace API
 	void bring_top(window, bool activated);
 	bool set_window_z_order(window wd, window wd_after, z_order_action action_if_no_wd_after);
 
+	void draw_through(window, std::function<void()>);
+	void map_through_widgets(window, native_drawable_type);
+
 	nana::size window_size(window);
 	void window_size(window, const size&);
 	bool window_rectangle(window, rectangle&);

include/nana/gui/widgets/widget.hpp

@@ -401,6 +401,16 @@ namespace nana
 		{
 			return *scheme_;
 		}
+
+		void draw_through(std::function<void()> draw_fn)
+		{
+			API::draw_through(handle(), draw_fn);
+		}
+
+		void map_through_widgets(native_drawable_type drawable)
+		{
+			API::map_through_widgets(handle(), drawable);
+		}
 	protected:
 		DrawerTrigger& get_drawer_trigger()
 		{