libcupsfilters: Let raster driver functions use log function

libcupsfilters: Let raster driver functions use log function

libcupsfilters contains a lot of functions for raster drivers, to
convert raster formats, handle colors with lookup tables (LUTs),
... See cupsfilters/driver.h for the API. These functions, originally
designed for the use in CUPS filters, had logged directly to
stderr. Now to allow easy use of them in filter functions we have
changed all logging to go into a provided filter function and
appropriately added parameters to the call schemes of the functions
which need it to supply the log function.

Also updated the printer drivers rastertopclx and rastertoescpx and
also the test programs testcmyk and testdither to use the changed API.