libppd: Make driverless printing PPDs from CUPS work correctly

libppd: Make driverless printing PPDs from CUPS work correctly

If one sets up a driverless printer with CUPS, either manually by the
"-m everywhere" option for "lpadmin" (or equivalent functionality in
the web interface) or automatically when CUPS creates a temporary
queue, the generated PPD file did not work correctly with current
cups-filters.

CUPS' auto-generated PPD files to not contain "*cupsUrfSupported: ..."
or "*cupsPwgRaster..." lines, but instead, add pseudo-PostScript code
(as in PPDs for CUPS Raster drivers) to the "ColorModel" and
"cupsPrintQuality" options.

Therefore the ppdLoadAttributes() function should not create artifical
"urf-supported" or "pwg-raster-..." printer IPP attributes and the
ppdFilterLoadPPD() should instead create a sample Raster header as it
already does for PPDs for CUPS Raster drivers. This way color space,
depth, and resolution get supplied to the filters via the header.

This commit does the appropriate changes.