Add spectral blending mode

If it is possible to find spectral data with a smaller number of channels it should solve the performance problem of the smudge brush, except that the spectral blending of the colour picker with the colour sample of the smudge brush has not yet been implemented

This is the thread in KA: https://krita-artists.org/t/i-realized-the-spectral-mixing-of-mypaint-in-krita/33473/137