libkwineffects/kwinglutils: handle alpha better with colormanagement

Alpha from clients is encoded in electrical values, and some of our code also relies on compositing results being encoded in electrical values. To take care of this, we need to undo the premultiplication of rgb values, convert to or from linear, and then re-do the premultiplication. This doesn't result in exactly the same blending behavior with linear blending, but it's close enough