Some methods that change state use &self instead of &mut self about freetype-rs

[andrewrk]

Mutability is referring to whether any of the fields of face need to change when that method is called. Although some internal state might change in the freetype library code, the Face struct itself does not need to change.

[nstoddard]

Some projects such as glfw-rs use &mut self even for methods that don't modify struct fields. See PistonDevelopers/glfw-rs@6b5c2fe for an example. It seems like we should follow that example. For one thing, it's conceivable that at some point we might want to change some of those methods so that they do change fields, which would require switching to &mut self anyway. I'd rather not risk having to break the API like that.

[bvssvni]

I asked in #rust-internals about this, and was told the calling order of FFI functions won't be affected.

[bvssvni]

This seems like an issue similar to when to use unsafe, which also has grey areas in library design. I can't tell which way is right, because there could be different contexts where either case is wrong. The argument in favor of using &mut self is that the API breaks less code when doing &mut self => &self, than &self => &mut self. If we suspect we will run into this issue and there is no context we can think of when &self is better, then I think we should use &mut self.