libcupsfilters: In PPD generator create only one *cupsFilter2: line for raster

libcupsfilters: In PPD generator create only one *cupsFilter2: line for raster

If a printer supports more than one raster format (Apple Raster, PWG
Raster, PCLm), create only one "*cupsFilter2: ..." line for raster,
for the most desirable/reliable raster format.

Most preferred is Apple Raster, then PWG Raster, and after that PCLm

See

    https://github.com/OpenPrinting/cups-filters/issues/498

Here a printer works correctly with Apple Raster but does not print
the page in its original size with PWG Raster.