[unity] Sequence animations incorrectly render with threaded mesh generation

Reported on the forum here:
https://esotericsoftware.com/forum/d/29454-43-beta-multithreaded-sequence-animation-issue/5
The issue is that Attachment is shared state and a sequence attachment modifies the Attachment's UV region, which is not a problem until multiple skeletons with this Attachment set are processed on different threads.