Feature request: expose frame.backdrop as a bindable runtime texture in custom effect shaders, alongside frame.input.

I’d like to have a backdrop filter like saturate, but in order to do that, I need the shader to be able to receive the layer’s own rendered content, as well as use backdrop data to alter it. 

So, I’d like to request the shader effects to have read access to the already composited backdrop buffer for sampling, colour space, clipping, transforms and performance constraints.