Just discovered that the clipping in the new layer modes needs more work.

It's not a problem with colors that originated in sRGB, but the layer 
modes can easily construct colors outside sRGB, or even 
invisible/virtual colors.
The current behavior is different from floating point/GEGL implementation.

Especially when composing e.g. in Color mode onto a white layer, results 
differ. A red onto white will become yellow (which isn't necessarily 
worse than the red from floating point, as it seems to be closer to 
correct lightness -- but it's different.)

The problem is that I have to clip twice in integer mode: once from Lab 
to XYZ, then again from XYZ to RGB.

Widening the lab-to-xyz LUT solved the problem for red, but created 
strange color bandings for blue, so it probably leads to integer 
overflow somewhere.

In other words: I'll have to work on the lab-to-rgb conversions...

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