Use const for std::string arguments.
Set default font and add loading of additional fontconfig config file.
Add /Library/Fonts for MacOS.
Remove unused path lookup and move OPENSCAD_FONT_PATH to fontconfig.
Show font path in Library Info window.
Fix render issue with combining chars intersecting with the base char.

Example: text("A\u030A", font = "Arial");

The fix relies on the winding order of the polygons reported by FreeType
to be correct, so the Clipper union operation keeps that information.
Merge pull request #866 from openscad/text-fixes

Text fixes