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.
Maniphest Tasks: T85261
Differential Revision: https://developer.blender.org/D10286
85fe12071a