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

[Artur Malabarba]

I have the following snippet in my init file.

(set-face-attribute 'default nil :height 100
                    :font "SourceCodePro Medium")
(set-fontset-font "fontset-default" nil
                  (font-spec :size 40 :name "Symbola"))

On my Ubuntu system, this means any characters no supported by the
main font (SourceCodePro) will displayed in the Symbola font and
considerably scaled up in size (good).

On my Arch GNU/Linux system, the same happens, except the Symbola
characters are not scaled up in size. By inspecting them with C-u C-x
= I can verify that the Symbola font is indeed being used:

   xft:-unknown-Symbola-normal-normal-semicondensed-*-13-*-*-*-*-0-iso10646-1
(#x1BE3)

Both are compiled from master.