Resolve linked fonts in fontManager.findfont()

The commit ensures that the realpath of linked fonts is used across the library.
This prevents embedding linked fonts twice and makes sure that linked fonts are
referenced consistently in PDF files. The link resolution cannot happend in
addfont() since this would break the *directory* argument of findfont().

Author: sauerburger

lib/matplotlib/font_manager.py

@@ -1238,9 +1238,10 @@ def findfont(self, prop, fontext='ttf', directory=None,
         rc_params = tuple(tuple(rcParams[key]) for key in [
             "font.serif", "font.sans-serif", "font.cursive", "font.fantasy",
             "font.monospace"])
-        return self._findfont_cached(
+        filename = self._findfont_cached(
             prop, fontext, directory, fallback_to_default, rebuild_if_missing,
             rc_params)
+        return os.path.realpath(filename)
 
     @lru_cache()
     def _findfont_cached(self, prop, fontext, directory, fallback_to_default,