Make attributes on all pub structs public

I've been having some problems after my latest upgrade, since a lot of struct attributes that were public before are private now (due to the changes in Rust). To solve this problem, I went ahead and made all attributes on all public structs public.

I'm not sure if this is a 100% correct, since at least some of the FreeType functions look like they access data from structs, but decided to go ahead anyway, for the following reasons:
- The attributes were public before the change in Rust, so nothing really changes (although I haven't checked the history in detail, there might have been private attributes before).
- Since these are low-level bindings to a C library, I don't think restricting access is the right thing to do anyway.