Fix crash on double cleanup of GL canvas back-end.
Fix crash on double cleanup of GL canvas back-end.

Ignore-this: a3dc86a4edc810dabcc537500551c1bd

darcs-hash:20141021224229-4d2ae-0e4d8a48d267d5c8fe346a5f321463cdf1f9e9da

johntyree/HsQML,johntyree/HsQML