Groups: pass *in* instead of pass through

[Fotoloco]

It would be very handy if we could make groups that take input from lower layers (like with the pass through blend mode) while allowing selection of any blend mode for the group. For example, you could use a pixel layer inside the group to apply an overlay blend, and then set the blend mode for the group to luminosity if the shift in colors/saturation is undesired. I use tricks like this all the time but currently I have to flatten the layers below and insert them into the group, which prohibits dynamic editing of those layers.