Updating skeletons in parallel is not thread safe

[jabuwu]

I'm making a note of this here in case anyone tries it. It does not work. The primary suspects seems to be reference counting of attachments being non-atomic.

rusty_spine/src/attachment.rs

Line 29 in 34a3264

(*(c_attachment as *mut spAttachment)).refCount += 1;

Unfortunately, this is a non-trivial fix since the C code won't play nice with Rust atomics.

Skeletons which do not share attachments (skeletons that have a different SkeletonData as others) are probably fine to be updated in parallel.

Attachments in general are really unsafe and could use a proper Handle type to address them safely.