bug#31288: 27.0.50; [PATCH] Make cairo build somewhat usable

[Eli Zaretskii <eliz@gnu.org>]

> From: Ari Roponen <ari.roponen@gmail.com>
> Date: Fri, 27 Apr 2018 16:19:16 +0300
> 
> Attached are two patches to make --with-cairo builds more usable.
> 
> First patch tries to fix screen corruption, and the second patch allows
> cairo builds to display images.

Thank you for working on this.

Please allow me a couple of minor comments on the changes:

> @@ -1236,7 +1231,8 @@ x_update_end (struct frame *f)
>      {
>        cairo_t *cr = 0;
>        block_input();
> -#if defined (USE_GTK) && defined (HAVE_GTK3)
> +      /* FIXME */
> +#if 0 && defined (USE_GTK) && defined (HAVE_GTK3)
>        if (FRAME_GTK_WIDGET (f))
>          {
>            GdkWindow *w = gtk_widget_get_window (FRAME_GTK_WIDGET (f));

This hunk doesn't look right.  If the code in question causes some
trouble to a Cairo build, please ifdef it out only when USE_CAIRO is
defined, not unconditionally.  And if that still needs a FIXME
comment, please add some description of what needs to be fixed and
why.

> +      cairo_surface_destroy (s);
> +    } else {
> +    SET_FRAME_GARBAGED (f);

Please use GNU style of braces.

Thanks again.