Enable contrast control in all modes of tone equalizer

[kofa73]

Is your feature request related to a problem? Please describe.
In tone equalizer's guided filter and eigf modes, a mask contrast compensation control is available. It can be used to fit the tonal range in the -8 EV .. 0 EV scale. The original intent was to counter the contrast reduction caused by blurring the mask; however, one can find examples of pixls.us where people actually use it to 'zoom into' a part of the tonal range they want to manipulate, to obtain finer control.

Since contrast reduction is not so severe when preserve details is set to one of the averaged modes and is not present at all with no, the contrast control is hidden, and the code uses 1.0 for the value (see https://discuss.pixls.us/t/processing-skies-with-clipped-channels-in-darktable-filmic-sigmoid/28995/105 by Aurélien). However, people would still like to be able to 'zoom in', see https://discuss.pixls.us/t/processing-skies-with-clipped-channels-in-darktable-filmic-sigmoid/28995/68.

Describe the solution you'd like
Enable the contrast control in all modes -- https://discuss.pixls.us/t/processing-skies-with-clipped-channels-in-darktable-filmic-sigmoid/28995/103

Alternatives

• use one of the modes where this is already available
• 'darken the sky with tone EQ' + 'mask the sky in color balance RGB, set the white fulcrum properly, darken on the power channel and increase the linear global chroma by a lot' (https://discuss.pixls.us/t/processing-skies-with-clipped-channels-in-darktable-filmic-sigmoid/28995/38 by Aurélien)

Additional note
It is my understanding that simply enabling the control could break existing edits: if someone with the current code tries several modes, first trying a guided filter, sets a contrast enhancement, then switches to one of the modes where the contrast adjustment is not used, a hard-coded value of 1.0 is applied by the formula. By naïvely modifying the code, when reprocessing the image, we’d suddenly apply the contrast adjustment read from config, originally set for the guided filter, and thus the mask and the effect would change.

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