Improve some NS drawing code

Improve some NS drawing code

* src/nsterm.m (ns_update_end): There's no need to schedule a redraw
if nothing has been changed.
(ns_set_vertical_scroll_bar):
(ns_set_horizontal_scroll_bar): Fix the logic for clearing under the
scrollbars.
(ns_clear_under_internal_border): No need to clip, the default
clipping rectangle will be fine.