Add filter or option to Import Image Stack to pad images to a certain Width/Height #159

This could be automated so the filter gets the largest dimensions of width and height by looking at all the images first, then padding the images to that maximum width/height, then finally doing the import.
This would require the preflight to examine each image in the series for its width/height which might slow down the preflight significantly for large image series. This slow down needs to be weighed by the benefits of adding the options. Maybe as a checkbox option and with some caching of the images the first preflight might take some time but after that it should be fast as long as the files are not changed.