patch 8.0.1002: unnecessarily updating screen after timer callback

Problem:    Unnecessarily updating screen after timer callback.
Solution:   Check if calling the timer sets must_redraw.
diff --git a/src/terminal.c b/src/terminal.c
index c522613..a8c7ae5 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -669,7 +669,7 @@
 	    update_cursor(term, TRUE);
 	}
 	else
-	    redraw_after_callback();
+	    redraw_after_callback(TRUE);
     }
 }