libcupsfilters: rastertopclm() fails if no default resolution given

libcupsfilters: rastertopclm() fails if no default resolution given

Some PCLm printers, especially the ones which support only one
resolution in PCLm ("pclm-source-resolution-supported" IPP attribute)
do not have the "pclm-source-resolution-default" IPP attribute. This
broke rastertopclm().

This is fixed now for both direct use of the printer IPP attributes or
use via an auto-generated PPD file. The missing default resolution is
simply filles with the first item of
"pclm-source-resolution-supported" then. If this item is also missing,
we error out on this printer/PPD file.