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/main.c b/src/main.c
index 877adaf..9cf10bd 100644
--- a/src/main.c
+++ b/src/main.c
@@ -1242,7 +1242,11 @@
 	    if (VIsual_active)
 		update_curbuf(INVERTED);/* update inverted part */
 	    else if (must_redraw)
+	    {
+		mch_disable_flush();	/* Stop issuing gui_mch_flush(). */
 		update_screen(0);
+		mch_enable_flush();
+	    }
 	    else if (redraw_cmdline || clear_cmdline)
 		showmode();
 	    redraw_statuslines();
@@ -1283,11 +1287,13 @@
 			|| conceal_cursor_line(curwin)
 			|| need_cursor_line_redraw))
 	    {
+		mch_disable_flush();	/* Stop issuing gui_mch_flush(). */
 		if (conceal_old_cursor_line != conceal_new_cursor_line
 			&& conceal_old_cursor_line
 						<= curbuf->b_ml.ml_line_count)
 		    update_single_line(curwin, conceal_old_cursor_line);
 		update_single_line(curwin, conceal_new_cursor_line);
+		mch_enable_flush();
 		curwin->w_valid &= ~VALID_CROW;
 	    }
 # endif
@@ -4212,11 +4218,7 @@
     /* A client can tell us to redraw, but not to display the cursor, so do
      * that here. */
     setcursor();
-    out_flush();
-#ifdef FEAT_GUI
-    if (gui.in_use)
-	gui_update_cursor(FALSE, FALSE);
-#endif
+    out_flush_cursor(FALSE, FALSE);
 
     return res;
 }