ImageSource/ImageReader -- improvement proposal

[radpopl]

1. "The first file in the sequence, i.e., corresponding to start, MUST exist in order for clip parameters to be computed. Any missing files in the sequence are replaced with a blank frame."

It would be nice if the first frame weren't required, and the clip parameters were taken from the first existing one.

2. It would be even better if you could pass the clip as a parameter, in which case the clip frames would be replaced (if there's an image on disk) or passed through unchanged (if there's no image on disk). This can supposedly be done through ConditionalFilter, but I think I've had problems with it before when using Prefetch.