add DPI_AWARENESS_CONTEXT_UNAWARE_GDISCALED define

src/video/windows/SDL_windowsvideo.h

@@ -115,11 +115,14 @@ DECLARE_HANDLE(DPI_AWARENESS_CONTEXT);
 
 /* Windows 10 Creators Update */
 #if NTDDI_VERSION < 0x0A000003
-
 #define DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2 ((DPI_AWARENESS_CONTEXT)-4)
-
 #endif /* NTDDI_VERSION < 0x0A000003 */
 
+/* Windows 10 version 1809 */
+#if NTDDI_VERSION < 0x0A000006
+#define DPI_AWARENESS_CONTEXT_UNAWARE_GDISCALED ((DPI_AWARENESS_CONTEXT)-5)
+#endif /* NTDDI_VERSION < 0x0A000006 */
+
 typedef BOOL  (*PFNSHFullScreen)(HWND, DWORD);
 typedef void  (*PFCoordTransform)(SDL_Window*, POINT*);