bug#19117: 25.0.50; emacs on x11 chooses different fonts for the same face sometimes

[Dima Kogan <dima@secretsauce.net>]

Hi. I looked at this some more. The scalable-fonts-allowed is indeed
being set to t in realize_basic_faces().

As emacs runs, the font lists are generated (xfont_list() called)
multiple times, and there are many code paths that end up doing that.
Some code paths go through realize_basic_faces(), and thus allow
scalable fonts, and some code paths do NOT go through that function.
Each time a new emacs frame is created, xfont_list() is called multiple
times, and if the last time it is called, the code path went through
realize_basic_faces(), then I get the wrong fonts chosen for that frame.
This is not consistent, so sometimes I see the bad behavior.

What's the best route from here? Options:

1. Look at all the (very numerous, and complicated) code paths, and set
it up such that realize_basic_faces() is never the last one. I don't
know enough to do this (yet), and can't tell if this is even appropriate

2. Do not set scalable-fonts-allowed to t in realize_basic_faces(). Does
anybody know why this is desirable?

3. Never choose scalable X fonts, regardless of scalable-fonts-allowed.
If the X scalable fonts are simply scaled bitmaps, then the scaled
result will never look particularly good, so choosing these is not
useful. This almost certainly doesn't apply to the other font backends
(xft, etc)