This removes any kind of scaling from the specific implementations, they
now truly report the bare minimum size needed, as well as a corresponding
image. AbstractBarcode scales this to the requested size, and, for now,
adds the previous magic numbers to the minimum size.

This is a further step towards allowing applications full control over
scaling for properly handling high DPI scenarios.

There's two noteworthy behavior changes in this:

• minimumSize() now also works before calling toImage() for the first time.
• toImage() returns valid results down to the actual minimum size, not only

to what minimumSize() reports.