load_network: add Powershell fallback for Windows

[amyspark]

This is useful for systems where libcurl isn't available at build time.

See dtschump/gmic#374

See https://krita-artists.org/t/unknown-filename-gmic-qt/37813

[dtschump]

Thanks for the two PR @amyspark !

[dtschump]

Hi @amyspark , I've added a new option user_agent in the cimg::load_network() function. Could you please check that I did it right for powershell ? Basically, I've done something like this:

#if cimg_OS==2
      if (cimg::fsize(filename_local)<=0) { // Try with 'powershell' otherwise.
        if (timeout) cimg_snprintf(s_timeout.assign(64),64,"-TimeoutSec %u",timeout);
        else s_timeout.assign(1,1,1,1,0);
        if (referer) cimg_snprintf(s_referer.assign(1024),1024,"-Headers @{'Referer'='%s'}",referer);
        else s_referer.assign(1,1,1,1,0);
        if (user_agent) cimg_snprintf(s_user_agent.assign(1024),1024,"-UserAgent \"%s\"",user_agent);
        else s_user_agent.assign(1,1,1,1,0);
        cimg_snprintf(command,command._width,
                      "\"%s\" -NonInteractive -Command Invoke-WebRequest %s %s -OutFile \"%s\" -Uri \"%s\"",
                      cimg::powershell_path(),s_timeout._data,s_referer._data,filename_local,
                      CImg<char>::string(url)._system_strescape().data());
        cimg::system(command,cimg::powershell_path());
      }
#endif

[amyspark]

@dtschump it looks correct! 👍