libcupsfilters, mupdftoraster, mupdftopwg: Renamed filter function

libcupsfilters, mupdftoraster, mupdftopwg: Renamed filter function

Renamed the filter function mupdftoraster() and the corresponding CUPS
filter to mupdftopwg(). This is because the "mutool" utility of MuPDF
only supports PWG Raster output, no CUPS Raster and no Apple Raster.

It also does not make the impression that this will change anytime
soon, as this is already this way for many years and there is also no
feature request posted asking for adding the other formats.

This way it is easily communicated that the filter function has to be
followed by the pwgtoraster() filter function to get CUPS Raster.

Note that the "outformat" parameter (the CUPS filter feeds in the
value of the FINAL_CONTENT_TYPE environment variable here) does not
change the actual output format (PWG Raster) of mupdftopwg(), it is
only to tell the final output format of the print job, so that
mupdftopwg() can grab color space, color depth, and resolution info
for this final format from the PPD file (or from the printer IPP
attributes) and call "mutool" according to this format.