When checking ranges in tty_cmd_cells, cannot use the tty cursor

When checking ranges in tty_cmd_cells, cannot use the tty cursor
position and tty_cursor because it may be at the final invisible cursor
position on automargin terminals. The text to be drawn is confined to
the pane, so use the pane cursor position for the checks instead. Fix
from Anindya Mukherjee, redraw problem reported by naddy@.