This fixes a regression introduced by commit
2a7a2371.

The frameworks expect the original thread to run its run loop, so input
sources (like distributed notifications) get processed.  For example,
Core Audio on Snow Leopard doesn't track changes in the default ouput
device, such as when headphones are plugged in, without this.

Otherwise we could end up with surfaces that have SFLAG_INSYSMEM set, but no
actual system memory allocated. This was exposed by
3bbe658e.