anti-alias fix results in ugly scaled fonts

[Stephen Gildea <stepheng+emacs@gildea.com>]

At HEAD, I get an ugly scaled font for my sans-serif text.
I noticed it in MH-E, and you can reproduce it as follows:

    emacs -Q -fn fixed --eval "(buffer-face-set 'variable-pitch)"

Although the default font is specified on the command line as "fixed"
(from the misc-fixed X11 family of fonts), and the variable-pitch font
selected is not in the misc-fixed family, having "fixed" as the
default font affects the font selected for the "variable-pitch" face.
Which font it selects changed recently.

This bug seems to have been introduced by commit 6b1ed2f2c9,
"Fix antialias face attribute when text is scaled", 27 August.

In an Emacs built before that commit, the above command displays text
with a pretty sans-serif font.

That commit undoes some of the changes that addressed bug#17973.
Bug#17973 fixed display of misc-fixed fonts.  With that recent commit,
using misc-fixed fonts again breaks some faces.

 < Stephen