variant BackdropEffect

One thing a plate does to the desktop under it. They stack: a plate is blurred, then bent and lit, then grained, whatever order they are listed in. Named one of a list rather than fields of the plate because a message cannot grow — adding a field changes the layout every client built against the older stubs reads — and a new kind of effect can be added here as a new arm, which a compositor that does not know it skips.