image files don't support multiple threads

[ashinn]

Segfaults can arise if saving an image with more than one thread. Until recently this included the default images, but mysteriously only after 24 hours had passed! This was due to the background thread checking for new leap seconds once a day. Commit 479efcdc339831d0b66c9eb818622ba76f2f4312 avoids creation of this thread unless the relevant env var is specified, without which the thread was running a noop anyway, bypassing the problem.

The underlying issue is likely a problem with repositioning context objects.