GlyphPosition + GlyphInfo

Honestly I failed to find sn.exe on my Visual Studio also am not sure this method of marshalling is right, `20` is just `sizeof` hb_glyph_info_t and hb_glyph_position_t and I don't know how to retrieve it with cleaner. I am aware this design is more similar to the original C API but I don't know how to redesign in C# way, however with this information [porting](https://github.com/ebraminio/glcourse/blob/master/tut1.c) is easy, I hope you could some time to have a look.