pngminus: Expect all image transformations to be available in libpng

The pngminus programs use several PNG image transformations:
`png_set_expand`, `png_set_expand_1_2_4_to_8`, etc. (in png2pnm.c);
`png_set_packing`, `png_set_invert_mono`, etc. (in pnm2png.c).
The availability of all of these transformations in libpng is now
required at compile time.

On the topic of transformations, apply an unrelated fix to the use
of `png_set_gamma`.
2 files changed