Add general loop

[CWBudde]

Motivation
I want to build a zoom animation on a gigapixel image. In order to do so, I need to scale crop a single image depending on the frame index of the output video. However, I don't want to create a dummy video to do this with the video frame iterator.

Description
If I use the gigapixel PNG image as video source, it does not seem to work as the video has no actual length.

Instead, I would want a custom loop, where I could probably define the start and the end number ([frame] index).

Alternatives
If it's easier, I wouldn't mind if the 'Video Frame Iterator' would be expanded to handle this scenario as well.