add finaliser to destroy a specific device context when the state token is GC'ed
add finaliser to destroy a specific device context when the state token is GC'ed

Ignore-this: 1a593253e4461c9752255502822e4326
- as opposed to destroying the current context when GC'ing the IORef holding the state

darcs-hash:20100923002633-dcabc-1d403f791e8a1a72a0b885955a082b3cba45ff13.gz

robeverest/accelerate,sjfloat/accelerate,robeverest/accelerate,rrnewton/accelerate,wilbowma/accelerate,wilbowma/accelerate