MinGW (or Windows at general?!) somehow doesn't expect four-byte
characters in a string and this class fails miserably when freeing
memory. Replaced with std::vector<char32_t>, it's horrendously ugly
solution, but I hope this will be resolved soon with newer MinGW GCC
(4.7.2 fails).
The testing is now slightly more sloppy due to inability to not pass any
Font or GlyphCache object. But it is actually better from user point of
view, as it is now impossible to do that by accident.
Provides a way to convert font into different format (either with or
without contents of associated glyph cache) or import/export glyph cache
(i.e. to avoid recreating it from scratch every time).