patch 9.0.0734: cursor position invalid when scrolling with 'smoothscroll'

Problem:    Cursor position invalid when scrolling with 'smoothscroll' set.
            (Ernie Rael)
Solution:   Add w_valid_skipcol and clear flags when it changes.  Adjust
            w_skipcol after moving the cursor.

src/structs.h

@@ -3714,6 +3714,7 @@ struct window_S
     pos_T	w_valid_cursor;	    // last known position of w_cursor, used
 				    // to adjust w_valid
     colnr_T	w_valid_leftcol;    // last known w_leftcol
+    colnr_T	w_valid_skipcol;    // last known w_skipcol
 
     /*
      * w_cline_height is the number of physical lines taken by the buffer line