Access Unity's back buffer?

Hi, I'm hoping to compare the current frame to the last frame in OnRenderImage using an image effect shader.

Does Unity natively use a back-buffer, and if so, is there a way to access it in script? And if not, what is the best way to store and read the last frame in script? (Unity 2018.3 SRP)
(Note: I'm actually trying to compare a downscaled temporary rendertexture to the downscaled last frame, but I don't think it should make a difference.)