Fix RGBA brushes created in Krita 4.4 becoming darker in Krita 5.0
In Krita 4.x releases there was a bug that caused lightness adjustments to be applied to the brush twice. It happened due to the fact that copy-ctor called brushTipImage() virtual method instead of just copying the image itself.
In Krita 5 we should open these brushes in somehwat the same way. The problem is that we cannot convert the numbers precisely, because after applying a piecewice-linear function twice we get a quadratic function. So we fall-back to a blunt parameters scaling, which gives result that is just "good enough".