Improve drawing performance on macOS

Improve drawing performance on macOS

* configure.ac: Require IOSurface framework.
* src/nsterm.h: New EmacsSurface class and update EmacsView
definitions.
* src/nsterm.m (ns_update_end):
(ns_unfocus): Use new unfocusDrawingBuffer method.
(ns_draw_window_cursor): Move ns_focus to before we set colors.
([EmacsView dealloc]):
([EmacsView viewDidResize:]): Handle new EmacsSurface class.
([EmacsView initFrameFromEmacs:]): Remove reference to old method.
([EmacsView createDrawingBuffer]): Remove method.
([EmacsView focusOnDrawingBuffer]):
([EmacsView windowDidChangeBackingProperties:]): Use new EmacsSurface
class.
([EmacsView unfocusDrawingBuffer]): New method.
([EmacsView copyRect:to:]): Get information from the context instead
of direct from the IOSurface.
([EmacsView updateLayer]): Use new EmacsSurface class.
([EmacsView copyRect:to:]): Use memcpy to copy bits around instead of
using NS image functions.
([EmacsSurface initWithSize:ColorSpace:]):
([EmacsSurface dealloc]):
([EmacsSurface getSize]):
([EmacsSurface getContext]):
([EmacsSurface releaseContext]):
([EmacsSurface getSurface]):
([EmacsSurface copyContentsTo:]): New class and methods.