NS (darwin): mode-line flickering when pixel scrolling

[Keith David Bershatsky]

In a recent bug that has since been resolved, Alan Third noticed that the 
mode-line was flickering when scrolling with pixel-scroll-mode active.

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=36633#47

The following link is a screen recording of pre-Mojave compatibility 
(ac74...bf5d) and post-Mojave compatibility (7946...b5fa).  I am building Emacs 
on OSX 10.6.8.

https://youtu.be/cXRLYKwWrj8

In the screen recording, I evaluate the following snippet -- first using 
(ac74...bf5d) and lastly using (7946...b5fa).  I am only using the down mouse 
wheel to scroll the display in the screen recording.

(progn
  (find-library "simple")
  (pixel-scroll-mode 1))

MOJAVE COMPATIBILITY:
- commit 7946445962372c4255180af45cb7c857f1b0b5fa
- CommitDate: Fri Sep 28 20:23:46 2018 +0100

PRE-MOJAVE COMPATIBILITY:
- commit ac7421423cd977e96a8f06d12dc6abdbfcaabf5d
- CommitDate: Fri Sep 28 14:56:58 2018 -0400

Inasmuch as Alan T. is already aware of the issue, I thought Emacs Devel might 
be the appropriate place to post my findings.