bug#21640: 25.0.50; Fontset-font is never scaled

[Artur Malabarba]

> Is the configuration the same, including the font back-ends?

The init files are the same (and I see this problem from emacs -Q
too), but there are many differences in system configuration outside
Emacs (most of which I probably don't know).

I haven't configured font-backends. If I query for it in
frame-parameters I get `(font-backend xft x)'.
Swapping their order or removing the `x' makes no difference. Removing
the `xft' segfaults Emacs.

> In general, I think you are getting undefined behavior: you specify a
> fixed size of 40 pixels, but expect it to be a kind of "relative"
> size?  Is that documented somewhere?

No, I expect to be bigger than my regular font, which is much less
than 40 pixels. I can change it to any other number with no effect.
I tried using a float too, which should represent a size in points
according to the docstring, but there's still no difference.