Add lossless image format support for jpeg-compare

Thanks for nice tool.

It would be great if `jpeg-compare` could also read any lossless format like PNG or TIFF, using it as a "baseline". For example, I would like to compare the original image with two different derived JPGs. Now I have to convert PNG into JPG with quality=100 and use it as a "baseline".

Start at the jpeg-compare image-input path and trace how the baseline image is decoded. Check how PNG and TIFF lossless inputs can be handled without converting them to JPEG, then verify comparisons against two derived JPGs use the original lossless image as the baseline. Done means both formats are accepted and the existing JPEG comparison behavior remains intact.

Written by the indexing model from the issue text.