bug#34256: 27.0.50; Crash on draw_glyphs()

[Eli Zaretskii]

> From: Kaushal Modi <kaushal.modi@gmail.com>
> Date: Mon, 4 Feb 2019 11:03:37 -0500
> Cc: 34256@debbugs.gnu.org
> 
> Thanks for the patch!
> 
> I couldn't apply the patch, so had to apply it manually.
> 
> But it works!

Thanks, I've now pushed it to the master branch.

> I am looking forward to understand in plain terms what this fix did. 

What happened was that while the display engine was redisplaying the
buffer, font-lock was called to fontify some part of the buffer, and
that activated Org mode (because the buffer displayed diffs of an Org
file, I presume?), and Org mode explicitly clears the image cache of
the frame for some reason I couldn't fathom.  So an inline image that
was ready to be displayed got removed and freed from under the feet of
the display engine, and when the time came to actually display it,
there was nothing to display...

The fix is to inhibit clearing of a frame's image cache until after
the frame has been updated by the display engine.

Thanks for helping me track this bug.