Use dedicated flag for lossless compression

[stamminator]

Currently, --quality 0 is used to indicate lossless compression. I think it would be an improvement to instead use a dedicated -l, --lossless flag for the following reasons:

• It's more intuitive/readable
• It would be consistent with the GUI app
• It would allow for lossy compression to be done with a quality of 0. Using the Caesium GUI app, my quick testing with some PNG screenshots shows lossy quality 0 producing files that are less than half the size of lossy quality 1, and they still have way better text readability than JPGs of the same or even double the size, so there's a solid use case for PNG lossy quality 0 for people who want to archive screenshots with readable text at the minimum possible file size.

The rules should probably be that a command must have either the --lossless flag or the --quality option set, with --quality either being ignored when --lossless is set or resulting in an error being thrown.

The bad part about this is it would be a breaking change, requiring people's scripts to be modified and a version jump to 1.x if you plan to use semver.

[Lymphatus]

Having a dedicated --lossless flag was on my radar for a long time, but I basically had the same doubts you're pointing out.

I'm not too worried about backward compatibility, as we are still on a 0.x.y release, so breaking changes are expected.

Having 0 as the "lossless value" is a very legacy heritage from the early days of this tool, when 0 was not allowed for quality. Now it is - at least for JPEG, PNG and WebP - so I think your solution is viable. It requires libcaesium to be modified a little bit, but it's not a big deal.