Fix #defines controlling when NS port draws to offscreen buffer

Fix #defines controlling when NS port draws to offscreen buffer

* src/nsterm.h (NS_DRAW_TO_BUFFER): New definition.
* src/nsterm.m (ns_update_begin):
(ns_update_end):
(ns_focus):
([EmacsView updateFrameSize:]):
([EmacsView initFrameFromEmacs:]):
([EmacsView copyRect:to:]): Use new #define.