Replace global variables pixel_progress etc with Progress class

[d11mshepcar]

I've replaced four global variables with a class that is passed around to fix issues when encoding/decoding from multiple threads as per issue #517

[hakonrk]

Thank you @mshepcar for providing this patch! 👍 It fixed the problem I was having when trying to decode multiple FLIF images in parallel using threads (images came out monochrome for some reason). Makes me wonder why it has not yet been merged to master. Are there any known problems?

[artscoop]

Ping, this MR is one blocking point for adoption in a Python lib

[bjorn3]

Please note that FLIF is superseded by FUIF, which itself is superseded by JPEG-XL.

[artscoop]

Oh nice

[jonsneyers]

Thanks for the improvement - sorry to have been so negligent in maintaining FLIF.

Like @bjorn3 said, FLIF is superseded by JPEG XL. So any project currently using FLIF should consider switching to libjxl.