vim-patch:9.0.1772: Cursor may be adjusted in 'splitkeep'ed windows (#24811)

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: vim/vim#12509

16af913eee

Co-authored-by: Luuk van Baal <luukvbaal@gmail.com>

src/nvim/buffer_defs.h

@@ -1194,6 +1194,7 @@ struct window_S {
   int w_hsep_height;                // Number of horizontal separator rows (0 or 1)
   int w_vsep_width;                 // Number of vertical separator columns (0 or 1).
   pos_save_T w_save_cursor;         // backup of cursor pos and topline
+  bool w_do_win_fix_cursor;         // if true cursor may be invalid
 
   int w_winrow_off;  ///< offset from winrow to the inner window area
   int w_wincol_off;  ///< offset from wincol to the inner window area