CUPS filter not working

[andersonls]

The provided CUPS filter is not working properly.

The grf = GRF.from_pdf(pdf.read(), 'CUPS') on line 22 returns a list, so the grf.optimise_barcodes() and grf.to_zpl calls doesn't work.

What i did was change to always get the first element, but i'm not sure how it would work when printing multiple pages.