patch 8.1.0822: peeking and flushing output slows down execution

Problem:    Peeking and flushing output slows down execution.
Solution:   Do not update the mode message when global_busy is set.  Do not
            flush when only peeking for a character. (Ken Takata)
diff --git a/src/proto/screen.pro b/src/proto/screen.pro
index b78796e..0657831 100644
--- a/src/proto/screen.pro
+++ b/src/proto/screen.pro
@@ -49,6 +49,7 @@
 int win_del_lines(win_T *wp, int row, int line_count, int invalid, int mayclear, int clear_attr);
 int screen_ins_lines(int off, int row, int line_count, int end, int clear_attr, win_T *wp);
 int screen_del_lines(int off, int row, int line_count, int end, int force, int clear_attr, win_T *wp);
+int skip_showmode(void);
 int showmode(void);
 void unshowmode(int force);
 void clearmode(void);