patch 9.0.0206: redraw flags are not named specifically

Problem:    Redraw flags are not named specifically.
Solution:   Prefix "UPD_" to the flags, for UPDate_screen().
diff --git a/src/screen.c b/src/screen.c
index 6929501..01a6902 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -2906,7 +2906,7 @@
     screen_Rows = Rows;
     screen_Columns = Columns;
 
-    must_redraw = CLEAR;	// need to clear the screen later
+    must_redraw = UPD_CLEAR;	// need to clear the screen later
     if (doclear)
 	screenclear2();
 #ifdef FEAT_GUI
@@ -3028,8 +3028,8 @@
     win_rest_invalid(firstwin);
     redraw_cmdline = TRUE;
     redraw_tabline = TRUE;
-    if (must_redraw == CLEAR)	// no need to clear again
-	must_redraw = NOT_VALID;
+    if (must_redraw == UPD_CLEAR)	// no need to clear again
+	must_redraw = UPD_NOT_VALID;
     compute_cmdrow();
     msg_row = cmdline_row;	// put cursor on last line for messages
     msg_col = 0;
@@ -3652,7 +3652,7 @@
 {
     while (wp != NULL)
     {
-	redraw_win_later(wp, NOT_VALID);
+	redraw_win_later(wp, UPD_NOT_VALID);
 	wp->w_redr_status = TRUE;
 	wp = wp->w_next;
     }