* End of the pImpl refactoring task started in [972]. FTGlyph was the last
class needing the change. As a consequence, FTGlyph is now also exported
in the library API, and so are FTBBox and FTPoint.
* Move FTGlyph, FTFont and FTLayout classes and their derivatives into
separate subdirectories of src/ to avoid cluttering src/ with too many
files. The Visual Studio solution still needs an update.