patch 9.1.1458: tabpanel: tabs not properly updated with 'stpl'

Problem:  tabpanel: tabs not properly updated with 'stpl'
Solution: remember the Column offset per tabpage
          (Hirohito Higashi)

fixes: #17519
closes: #17544

Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/structs.h b/src/structs.h
index 55181eb..250dc2f 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -3654,6 +3654,7 @@
     long	    tp_old_Rows;    // Rows when Tab page was left
     long	    tp_old_Columns; // Columns when Tab page was left, -1 when
 				    // calling shell_new_columns() postponed
+    int		    tp_old_coloff;  // Column offset when Tab page was left
     long	    tp_ch_used;	    // value of 'cmdheight' when frame size
 				    // was set
 #ifdef FEAT_GUI