Allow dynamic choice of drawing path on NS (bug#39883)

Allow dynamic choice of drawing path on NS (bug#39883)

* src/nsterm.h (NS_DRAW_TO_BUFFER): Let this be enabled on versions
older than 10.14.
* src/nsterm.m (ns_update_begin):
(ns_update_end):
(ns_focus):
(ns_unfocus):
([EmacsView viewDidResize:]):
([EmacsView createDrawingBuffer]):
([EmacsView windowDidChangeBackingProperties:]):
([EmacsView copyRect:to:]):
([EmacsView wantsUpdateLayer]): Dynamically switch between drawing to a
buffer and drawing to the screen, depending on the version of AppKit
in use.
([EmacsView dealloc]): We can't release the context unless
NS_DRAW_TO_BUFFER is defined.