add GFX2_Get_Window_Handle() to get the win32 HWND

and use it

src/screen.h

@@ -29,6 +29,9 @@
 #ifndef SCREEN_H_INCLUDED
 #define SCREEN_H_INCLUDED
 
+#ifdef WIN32
+#include <windows.h>  // for HWND
+#endif
 #include "struct.h"
 #include "global.h"
 
@@ -63,4 +66,8 @@ void Allow_drag_and_drop(int flag);
 void GFX2_UpdateScreen(void);
 #endif
 
+#if defined(WIN32)
+HWND GFX2_Get_Window_Handle(void);
+#endif
+
 #endif // SCREEN_H_INCLUDED