It's not implemented in GCC 4.9.2's stdlib (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57250). Instead, we're now always using a mutex to protect access; we previously created a mutex only on cancelation, but since we're always canceling now, it makes sense to allocate it right away.
This still has the unwanted side effect off blocking the Map thread
waiting for the workers to return (because they hold a reference
to the Map thread main loop).
Maybe the only way to solve this would be having a I/O thread for
dispatching messages.
The Request is not bound anymore to the invoker thread because the workers
will request for glyphs, but the reply will arrive at the Map thread.
