Decrease the preferred size for PDF417 barcodes

Unlike the other two-dimensional codes its smallest output element is not 1x1
but 1x3 pixels, so a scale factor of 2 already gives us enough area for barcode
scanners to work with, and avoids PDF417 barcodes becoming unreasonably large
by default.

The implementation is a bit hacky unfortunately, as surprisingly the base
class has no knowledge about the specific barcode type.
3 jobs for !10 with work/improve-pdf417-preferred-size in 34 seconds (queued for 6 seconds)
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Status Name Job ID Coverage
  Build
passed android_qt515 #147404
Linux

00:00:18

passed freebsd_qt515 #147405
FreeBSD

00:00:30

passed suse_tumbleweed_qt515 #147403
Linux

00:00:34