In everything in EEVEE_renderpasses_postprocess (or the corresponding
renderpass_postprocess_frag.glsl) colors get divided by a
'currentSample'.

This 'currentSample' is always incremented in
EEVEE_temporal_sampling_draw (and also one more time before we reach
EEVEE_renderpasses_postprocess.
This results in a "off-by-one", slightly inacurate colors and slight
inaccurate transparency (in certain passes like AOVs).

Now decrement the currentSample EEVEE_renderpasses_postprocess again
by one to compensate.