Maximum zoom level should be proportional to image size

I think the current maximum zoom level is a static value, independant from image size.

It's fine in most cases, but for larger images it may not allow enough precision to draw some tiny lines.

Two options:

1. try to make the max zoom level dependant on image size (while keeping the current max as the lowest possible vlaue)
2. simply extend that value to allow further zoom

Refs https://gitlab.com/arkindex/requests/-/issues/444 (with some images)