The Worker now always waits for subprocesses. When faced with multiple
errors (like I/O reading from stdout and a returned error status from
the process) will return the most important one (in this case the exit
status of the process).
Subprocesses need to be waited for, even when they crashed, otherwise
they will linger around as "defunct" processes. This caused
out-of-memory errors, because several defunct Blenders were eating up
the memory.
Blender and FFmpeg were run in the same way, using copy-pasted code. This
is now abstracted away into the CLI runner, which in turn is moved into
its own subpackage.
No functional changes.
