I would love to have a shader effect (or I guess blend mode) that would add the alfa of a foreground image to the background. (and otherwise ignore the foreground image).
Just like the destination in blend mode, only the alfa of the foreground image is not subtracted from the luminosity of the background, but added to the alfa of the background (making it transparent instead of black)
It would be extremely useful for solving problems like the one I'm describing here: