[cups] Auto-reformatting shipping labels for a 4x6 thermal printer

I started doing small-scale e-commerce projects, and found that manually
feeding US Letter (8.5x11) sticky labels through my old LaserJet 5L was a
pain.  So I bought a surplus Zebra GK420d thermal label printer, plugged it
into my network, and loaded it up with a spool of 4x6 shipping labels.  The
Zebra printer speaks ZPL and works flawlessly with CUPS.  I have been very
happy with it so far.

This arrangement is fairly painless when an online service (e.g. FedEx)
provides a PDF with a label specifically formatted for 4x6 label printers.
It's more of a hassle when they only want to provide PDFs sized for US
Letter printers, with 2 labels per sheet (or 1 label per half sheet).  If
you purchase bulk shipping labels from eBay, for instance, there is no
option to request 4x6 labels; they just send you one giant PDF with 2
labels per 8.5x11 page.  If you try to print this to the 4x6 Zebra printer,
the labels get completely mangled; usually it gets cut off right in the
middle.

The workarounds I found searching online involved manually importing each
bulk label PDF into Adobe Acrobat and cropping it by hand.  This isn't a
very scalable solution and there ought to be a way to make the software do
the heavy lifting for us.

What I'd like to do is have CUPS notice that I'm printing an 8.5x11 sheet
of labels to a 4x6 form factor printer, and automatically reformat each
label on the page to fit the 4x6 media.  I would also like to have the
ability to optionally resize it to fit on smaller 4x3 labels, which are
more convenient for tiny packages.

What would be a good way to approach this?  I would be comfortable writing
some code if needed.  I don't know the CUPS architecture at all, though,
and I'm not sure where to start.

cc: Michael since he seems to be the main contributor
to filter/rastertolabel.c
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