Correct tpo scan area for agfa arcus 1200

Corrected the transparency adapter scan area for the agfa arcus 1200 scanner. Previously it was impossible to scan the whole area because it was limited to the default values (129 mm x 180 mm). According to the owner's guide the adapter's scan area is 203 mm x 254 mm.

This change was tested with my own agfa arcus 1200 scanner.