Code review comment for lp:~sil2100/compiz/fix_ftbfs_arm_boost

I was going to mention changing the boost::function signature in GLDoubleBuffer, but then I remembered that we didn't want to introduce a dependency on Display * within GLDoubleBuffer and DoubleBuffer generally, so passing it as a non-variable parameter to fufill the bound function signature is perfectly fine.

It is tested btw, by the DoubleBuffer tests :)