fix spngsave conversion
fix spngsave conversion

It was all mixed up. We don't need to call colourspace -- this is done
for us by the SAVEABLE system.

Fixes "vips identity x.png" error, see:

https://github.com/libvips/libvips/pull/2808#issuecomment-1133947162

lovell/libvips,lovell/libvips,lovell/libvips,lovell/libvips