patch 9.0.1772: Cursor may be adjusted in 'splitkeep'ed windows

Problem:    Cursor is adjusted in window that did not change in size by
            'splitkeep'.
Solution:   Only check that cursor position is valid in a window that
            has changed in size.

closes: #12509

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Luuk van Baal <luukvbaal@gmail.com>
diff --git a/src/structs.h b/src/structs.h
index eefc8f2..eb8d30b 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -3778,6 +3778,7 @@
     int		w_vsep_width;	    // Number of separator columns (0 or 1).
 
     pos_save_T	w_save_cursor;	    // backup of cursor pos and topline
+    int		w_do_win_fix_cursor;// if TRUE cursor may be invalid
 
 #ifdef FEAT_PROP_POPUP
     int		w_popup_flags;	    // POPF_ values