CI passes `-D warnings` to clippy, so there it will be an error either way.
For local development it is nice not to block compilation of dependent
crates on lints though.
This changes `QrVerificationData::from_luma()` to accept any struct that
implements GenericImage and GenericImageView.
This way we don't force a specific container for the ImageBufffer, hence
the developer doesn't necessarily need to copy the image data into a
Vec<u8>.
