rework the final-scale and make it possible to have a sharpen after downscaling

For the final-scale there is discussion to happen here.

For the final sharpen there is also discussion to happen here.

I had this proposal:

• Introduce a new hidden module named finalsharpen
• Make this module disabled by default
• This new module has a single parameter strength to allow for setting the amount of sharpening (0 = None)
• A new slider "final sharpening" is introduced into the export module (0 by default) up to (100%)
• When this slider is > 0 during export the finalsharpen module is enabled
• The actual strength is passed to it using config key

This issue is to track down the discussion for the implementation of those two issues. Please keep this discussion focused.

EDIT: Initial discussion was in #13635.

EDIT: From reference, having the finalscale doing the job in linear data is a good goal for quality.

EDIT: Moving downscale before any tone curve (Sigmoid, Filmic...) means that all modules after that will work in down-scaled images, what is the impact on quality?