fix(api/buffer): fix handling of viewport of non-current buffer

A lot of functions in move.c only worked for curwin, alternatively
took a `wp` arg but still only work if that happens to be curwin.

Refactor those that are needed for update_topline(wp) to work
for any window.

fixes #27723
fixes #27720

src/nvim/fileio.c

@@ -3197,7 +3197,7 @@ void buf_reload(buf_T *buf, int orig_mode, bool reload_options)
     curwin->w_topline = old_topline;
   }
   curwin->w_cursor = old_cursor;
-  check_cursor();
+  check_cursor(curwin);
   update_topline(curwin);
   keep_filetype = false;