Most of the api is functionally thread-safe, but I don't think that any of it is guaranteed to be.  Notable problems are the various color value setting/getting functions, as they may use intermediate storage in the class to convert between colorspaces.  (so if those intermediates are 'clobbered' by another thread, the first thread won't get the result it expects -- i know this from experience, doh! :D)

As long as you're doing just "internal" work in your threads, then either synchronize or iterate through the individual thread containers one at a time after they've stopped for "rendering", then you'll likely be safe.  That is, I'd suggest trying to stay away from doing any unsynchronized "drawing" inside a running thread.