Allow flatten with encode

[tvdeyen]

Flatten (reducing an animated GIF) should be allowed during encoding images in other formats.

[markevans]

hi - cheers for this - the only thing I'm concerned about is now there could be an ever-growing whitelist that I don't want to support, because encode is mainly just for switching to a different format (png -> jpg) and people can easily do their own custom processors.
What's flatten used for exactly and are there many other arguments that have a similar importance that could also be added to the whitelist (again, something I don't want to support) - thanks

[tvdeyen]

We found a solution that works for us (AlchemyCMS/alchemy_cms#2106), so I thought it might be of interest for others.

But I absolutely understand that maintaining such list (can I suggest to rename it to Safelist for obvious reasons, btw.?) is not what you want to do.

Since mutating a constants seems a bit awkward, what about adding an extension point or configuration for adding arguments to that list?