Fix scrollbars on macOS 10.13 and below (bug#54623)

Fix scrollbars on macOS 10.13 and below (bug#54623)

Make any build on macOS 10.13 and below follow the same drawing path
as the GNUstep port.  macOS 10.14 and above will use EmacsLayer.

* src/nsterm.h (EmacsLayer):
* src/nsterm.m ([EmacsView makeBackingLayer]):
([EmacsView unlockFocus]):
([EmacsView windowDidChangeBackingProperties:]):
([EmacsView copyRect:to:]): Remove any code required for macOS 10.13
and below, and fix the #if's to enforce strict separation of the
drawing paths.
(ns_update_end):
(ns_unfocus): Fix #ifs so that flushWindow is called on old macOS
versions as well as GNUstep.