Support FT_Bitmap_Convert

[obsgolem]

The function is found here in the documentation. There is currently a comment at https://github.com/PistonDevelopers/freetype-rs/blob/master/src/bitmap.rs#L121 saying that it is possible to change the pitch with FT_Bitmap_Convert, when in reality the bindings do not allow this. Perhaps a convert function taking the new alignment and returning a new bitmap would work?