patch 8.0.1449: slow redrawing with DirectX

Problem:    Slow redrawing with DirectX.
Solution:   Avoid calling gui_mch_flush() unnecessarily, especially when
            updating the cursor. (Ken Takata, closes #2560)
diff --git a/src/proto/gui.pro b/src/proto/gui.pro
index baad47f..4004bc7 100644
--- a/src/proto/gui.pro
+++ b/src/proto/gui.pro
@@ -25,6 +25,9 @@
 void gui_write(char_u *s, int len);
 void gui_dont_update_cursor(int undraw);
 void gui_can_update_cursor(void);
+void gui_disable_flush(void);
+void gui_enable_flush(void);
+void gui_may_flush(void);
 int gui_outstr_nowrap(char_u *s, int len, int flags, guicolor_T fg, guicolor_T bg, int back);
 void gui_undraw_cursor(void);
 void gui_redraw(int x, int y, int w, int h);