tone_mapping: add dynamic knee point adaptation tuning

Based on how close the target knee point is towards the 'extreme' values (min/max knee), we raise the adaptation strength dynamically. Significantly improves visual appearance of very bright/dark scenes, especially the sulphur pools clip.

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merged

@haasn Hi! I'm currently looking through this implementation as i want to learn more about HDR->HDR tone mappings. It looks to me like this snippet has a flaw: pl_smoothstep(max_knee, def_knee, target) Shouldn't this be pl_smoothstep(def_knee, max_knee, target) instead? Greets..

@toxie No. The intent is to have a "bathtub curve", which goes to 0 at the lower and upper bounds, but has a value of 1 at def_knee. pl_smoothstep(max_knee, def_knee, target) is a function that is 0 when target is close to max_knee, and smoothly goes towards 1 as target moves towards def_knee.

Keep in mind that smoothstep(A, B, x) is valid for A > B, and in this case basically horizontally mirrors the overall shape of the curve.

@haasn Thanks a lot for the clarification, i guess i'm stuck too much in the GPU world, where smoothstep() has that case undefined (according to spec at least).

@toxie huh, indeed it is - that's strange, given that the pseudocode they provide for the specification would works fine for edge1 < edge0 also. In fact, that's exactly the code we use..