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

[Eli Zaretskii]

> From: Pip Cet <pipcet@gmail.com>
> Date: Sun, 23 Jun 2019 08:05:55 +0000
> Cc: 36337@debbugs.gnu.org
> 
> The code does indeed assume that it is passed a bool vector which is
> padded to a multiple of 8 bits per line, but doesn't verify that the
> bool vector it is passed indeed matches this format, so it displays
> past the end of the bool vector.
> 
> This is easy enough to fix by rewriting the bool vector, but that's
> potentially very slow (on debug builds), so maybe we shouldn't do
> that?
> 
> Does the attached patch work for you?

Thanks, but I really hope there's a more elegant solution.  If not,
maybe we should simply require that both width and height be an
integral multiple of 8 in this case.