Update of gimp-netpbm2gimp-plugins to Netpbm 10.94.01

This is an update of gimp-netpbm2gimp-plugins to correspond to the 30-Mar-2021 release of the Netpbm utilities. The pamhomography utility has been added, and ppmbrighten has been removed as it's been superseded by pambrighten. In addition, a number of bug fixes and feature enhancements that have been made upstream to various utilities and to the Netpbm libraries are incorporated into this release. See the Netpbm change log for a list of everything that's changed in Netpbm v10.94.01 (and all prior versions).