patch 8.2.2064: terminal: cursor is on while redrawing, causing flicker

Problem:    terminal: cursor is on while redrawing, causing flicker.
Solution:   Switch the cursor off while redrawing.  Always add the top and
            left offset to the cursor position when not done already.
            (closes #5943)
diff --git a/src/popupwin.c b/src/popupwin.c
index 61f447d..cfd2050 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -3699,6 +3699,9 @@
     int	    attr_scroll = 0;
     int	    attr_thumb = 0;
 
+    // hide the cursor until redrawing is done.
+    cursor_off();
+
     // Find the window with the lowest zindex that hasn't been updated yet,
     // so that the window with a higher zindex is drawn later, thus goes on
     // top.