bug#36337: 26.1; XBM images are sometimes not displayed correctly

[Eli Zaretskii]

> From: Pip Cet <pipcet@gmail.com>
> Date: Sun, 23 Jun 2019 16:26:16 +0000
> Cc: triska@metalevel.at, 36337@debbugs.gnu.org
> 
> My suggestion would be to expand `substring' to work on bool vectors,
> then building a vector of bool vectors and using the existing code for
> that case. Less code in image.c, plus a new utility function that
> might be generally useful.

Or maybe we should have a variant of make-bool-vector that accepts 2
dimension s instead of just one?

> (However, do we want to encourage people to use bool vectors?)

Why not?  Evidently, it's convenient in this particular use case.

> > If not,
> > maybe we should simply require that both width and height be an
> > integral multiple of 8 in this case.
> 
> Why would you require the height to be a multiple of 8?

You are right, width is enough.