libcupsfilters: Let imageto...() with print-scaling=none center correctly

libcupsfilters: Let imageto...() with print-scaling=none center correctly

In the imageto...() filter functions with print-scaling=none do the
math for centering the image on the page without the unprintable
margins, so that the image gets correctly centered relative to the
page, not to the printable area of the page.

As in imagetoraster we have to pass on a bitmap of the printable area
for CUPS Raster. we correct for the margins after calculating image
size and position without margins.