bug#32932: 27.0.50; render bugs on macOS Mojave

[Eli Zaretskii]

> Date: Thu, 1 Nov 2018 22:55:19 +0000
> From: Alan Third <alan@idiocy.org>
> Cc: Aaron Jensen <aaronjensen@gmail.com>, boris@d12frosted.io,
>       32932@debbugs.gnu.org
> 
> ns_get_glyph_string_clip_rect is a simple wrapper round
> get_glyph_string_clip_rects, so when asked for the clipping rectangle
> for a single glyph, it returns a rectangle covering the entire row.
> 
> Because we just mark it as dirty and come back to draw it later we do
> end up redrawing the entire row.

I think if you are basing the redisplay on marking portions dirty, you
need to include the same logic as in display_and_set_cursor and its
callers.