ppd-cache.c: Check for required attributes if URF or PWG Raster are found

Some devices have image/urf in document-format-supported, but is missing urf-supported if AirPrint support is turned off, which breaks PPD generation.

Check for attribute urf-supported when we are about to decide whether the printer uses AirPrint, so in case the device supports another driverless standard, we can use it for PPD generation.

Apply the same for PWG Raster too.

Fixes Fedora issue #2263053