Application can not run in background

Since commit 38986c14 (kmscube: add option to run for specified # of frames) we ca nrun the application for a given number of frames. When using in automated systems for testing, running in background can be hekpfull. Examples are will be EMC and climate chamber tests.

See https://gitlab.com/niebelm/kmscube/-/tree/run-in-background for a proposed solution.

Thank you, Markus Niebel