bug#32932: 27.0.50; render bugs on macOS Mojave

[Charles A. Roelli]

> From: Alan Third <athird@googlemail.com>
> Date: Thu, 4 Oct 2018 15:07:13 +0100
> 
> On Thu, 4 Oct 2018, 14:07 Aaron Jensen, <aaronjensen@gmail.com> wrote:
> 
>  I apologize if this is tracked elsewhere. The current master build with
>  the new drawRect rendering seems to occasionally paint a blank buffer.
>  As I move the point, the line the point is on fills in. I don't know how
>  to reproduce this consistently, it just happens with normal use.
> 
> Hi Aaron,
> 
> Does it only blank the whole frame, or does it do it to certain areas?
> 
> Can you send the output from compiling the .m files? 
> 
> Thanks.

FWIW, I saw the same thing on macOS 10.6 on a recent build of master
(with the new rendering approach), but was unable to produce the issue
again.