After discussing with upstream ags developers, it was indicated that
making the destructor virtual changes the vtable and could cause
issues with plugins used by old games. This is unlikely to be an
issue with ScummVM since we do not use the original plugin DLL, but
to be consistent with upstream code I am reverting the change here.
And furthermore there was no need to make the IAGSFontRenderer
destructor virtual, as long as we do make the destructor of the base
renderers in the AGSSpriteFont plugin virtual (so that the Clifftop
Games versions of the renderers are properly destroyed).