Allow CMYK colors in Mpdf Output

[rasteiner]

This PR allows the Mpdf Output function to pass along an optional fourth argument to SetDrawColor and SetFillColor, so that CMYK colors can be used for the QR Code.

[finwe]

Thanks for the PR.

The spread operator cannot be used for this to be compatible with mPDF in lower versions of PHP.

Also, a unit test for the change should be present (which would reveal the incompatibility mentioned).

[rasteiner]

The test setup seems to be configured only for PHP 5.6 onwards.

Argument unpacking was introduced in PHP 5.6, so that should work.
Therefore I'm not sure to which lower PHP version you are referring to. I have added a unit test, the GitHub action however seems to require a human approval to run the tests.