Blender Git Loki

Fix for running bake operator in background mode without a job.

The cache bake operator would always create a job in the window manager.
However, when running in background mode via a script, the operator can
not be modal and the script would return right after starting the job.
Then Blender is stopped and the baking job with it.

To prevent this the operator now only runs in a job when executed from
the invoke function (or when 'use_job' is set explicitly).