You, possibly, can achieve that using another approach, not dealing with shaders.
Store a stack of a few (1-3, for example) previous rendered scenes and render all of them with an appropriate alpha.
If all you have is several shapes/sprites which move, then you might just store a stack of their previous positions and render them several times, looping through them. (in other words, no need to store *all* data related to the scene in the stack, just a few positions vectors).
Of course, it might be overkill, just a guess.