bug#32932: 27.0.50; render bugs on macOS Mojave

[Eli Zaretskii]

> Date: Thu, 8 Nov 2018 16:17:15 +0000
> From: Alan Third <alan@idiocy.org>
> Cc: aaronjensen@gmail.com, 32932@debbugs.gnu.org, boris@d12frosted.io
> 
> > What exactly do you mean by WHENEVER REQUESTED?  As opposed to what
> > alternative?
> 
> At the moment expose_frame doesn’t draw anything if the frame or
> window has been marked as garbaged

AFAIR, that's a mere optimization, so if you want expose_frame to go
ahead and redraw on NS regardless of the frame's garbaged flag, it's
fine with me.

> (there may be other circumstances too).

The only other case is when the frame's face cache is empty, in which
case you won't be able to draw anything anyway.

There's a no-op return in expose_window, but I think its condition
cannot happen nowadays, it's a relic from when expose_frame could be
entered asynchronously from a signal handler.

> If expose_frame could draw the rectangle as it was before the
> frame/window was marked garbaged, that would also solve the problem.

Not sure what this means: you can only draw what's in the glyph
matrices, what was there before the garbaged flag was set is gone for
good.