GUI-Controlled Slides scaling and cropping

[MaZderMind]

The GUI could have Buttons to help with correcting broken Slide-Stream inputs. It could have an assistant that helps detect the Problem:

Which option describes the Problem with the Slides best?
a) The Slides are compressed
b) The Slides are stretched
c) The Slides are cropped

This could be implemented using Messages, brokered by the core to running input-scripts, which would allow to keep the separation between core and sources. See #25, especially https://github.com/voc/voctomix/issues/25#issuecomment-158857822