typo fix some missing event (de)registrations ability to capture one event multiple times (instead of only having one slot, we have groups and each group has an own slot, the default group is "default" so we are backward compatible

This demonstrates the use of PNG_USER_CONFIG and pngusr.h

To build a minimal write-only encoder, run
    gather.sh  # to collect needed files from pngminus, libpng, and zlib
    make -f makefile.std