Effects cannot be concatenated onto a "Convolution" effect

[gkjohnson]

I've recently been trying out r3f's postprocessing and noticed that some effects are not being merged where they could be. "ToneMappingEffect" is marked as a "convolution effect" at the moment (more on that in another issue) and due to this condition, any following effects cannot be concatenated into a single pass shader which can have meaningful performance impact especially on mobile, resulting in more full screen passes than necessary.

It looks like this change was made in #250 (cc @CodyJasonBennett) without a lot of reasoning as to why. Without any more information it seems like the changes in #250 should be reverted. If there are other issues this is causing then I think other solutions should be discussed rather than implicitly defaulting effects to use their own pass that don't need it.