[Cin] zimg 2.8

[Andrea paz]

@Stefan Thanks for the hint. I had done the copy/paste from
bld_appimage.sh which indeed reports the error (@Phyllis: better
correct this error?).
The compilation starts but stops with an error.

I did a new build with the 2 patches. This time the build happens
without any problems. The zscale plugin is present, but it keeps
getting the usual error when I change the color parameters.

PluginFVClient::process_buffer() F_zscale
  err: Generic error in an external library