GlyphPosition + GlyphInfo

Honestly I failed to find sn.exe on my Visual Studio and on Developer Command Prompt 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 some cleaner way but not defining private variables of them on csharp. I am aware this design is more similar to the original C API but I am not sure how a redesign in C# way would look like, however with this informations [porting](https://github.com/ebraminio/glcourse/blob/master/tut1.c) or using it on [this example](https://github.com/Robmaister/SharpFont/blob/master/Examples/Program.cs#L78) would be easy, I hope you could get some time and have a look.