Real image collections fail

[ideamonk]

while input and output images work fine even on bigger sizes,
it was an assumption that users would supply a folder with real small thumbnails as collection param.
If some one passes a directory full of real digicam pics as collection, in most cases it freezes the user's system.

Solution - precalculations based on source dimensions and zoom factor

• warnings when more than a tolerable number of big images are found in thumbnails.

[ideamonk]

something like "Okay I can't usually do it on my own, but if you have put me on a monster machine, go ahead with your own risk. continue? y/n"