sam2p calls the pngtopnm tool on the system

[pts]

If pdfsizeopt_libexec/png22pnm called by pdfsizeopt_libexec/sam2p fails, then pdfsizeopt_libexec/sam2p also calls the pngtopnm tool on the system:

sam2p: Error: Filter::PipeE: system() failed: (png22pnm -rgba /tmp/tmp__sam2p__16767_0 || (pngtopnm </tmp/tmp__sam2p__16767_0 && pngtopnm -alpha </tmp/tmp__sam2p__16767_0)) >/tmp/tmp__sam2p__16767_1

The expected behavior is that pdfsizeopt_libexec/sam2p never calls the pngtopnm tool on the system.

One possible way to fix this is integrating the functionality of png22pnm to sam2p.

Another option is replacing sam2p with imgdataopt (https://github.com/pts/pdfsizeopt/issues/51).