can use wrong Symbol font

[jwilk]

As reported in #39 and in Debian bug #528808, Poppler can use an incorrect Symbol font.
This is most likely to happen on Windows, and our Windows distribution ships with fonts.conf that prefers URW fonts, but:

• We should choose the right font even with stock fontconfig setup.

• This is not covered by the test suite.

[jwilk]

This can be used as a test case:

\input common

\pdfpagewidth 12pt
\pdfpageheight 12pt

\pdfmapline{psyr Symbol}
\font\symbol=psyr at 12pt
\symbol

\hskip1pt\relax
D

\end

With PostScript Symbol font, it renders as Δ (genus 1).
With Windows Symbol font, it renders as Ø (genus 2).

[jwilk]

This is not covered by the test suite.

Test added in 301cb17.