The parameter unnecessarily complicates the implementation, as it needs
to be reimplemented in _every_ plugin (and all current plugins have bug
in it).
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).
Implementation is moved into `do*()` private virtual functions, public
interface is doing additional sanity checks around them. Opening files
is by default done in base implementation, which loads the files into
memory and then calls function for opening raw data.
Added interface for opening multi-file fonts from raw data, default
implementation just calls single-file implementation.
Function for creating glyph cache converts the text from UTF-8 to UTF-32
and then calls the implementation, removing the burden from
reimplementing this in each plugin.
Added unit test for file and single data opening, bumped plugin version
to 0.2.
Vladimír Vondruš