Post-process filters should return all layers

[zerebubuth]

At the moment, post-process filters are passed all layers as a parameter, but return one layer (or None) which is inserted back into the layers list. Post-process filters can alter all layers, however, and that's useful sometimes. The interface should just be passing all the layers in, and returning all the layers. This is simpler and easier to understand when we want to modify multiple layers at once.