[track] size of image cutout should have maximum

[mtbannister]

Image cutout will attempt to make a single huge cutout when sources are within a distance. This is fine (and ideal for triplets etc) but it should have a more sensible maximum.
At present if only a single new image is available in the stack it will make a VAST cutout that encompasses both the discovery image and the new location.
Vast cutout also makes it nearly impossible to load useful comparison images.