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EEVEE: Volumetrics

When viewport samples are set to 1 simple scenes with volumetrics crash.
EEVEE volumetrics needs to init the post processing buffers. With recent
changes the need for post processing buffers are known after the cache
init. But they are constructed before the cache init. This lead to null
pointers.

Reviewed By: fclem

Maniphest Tasks: T64922

Differential Revision: https://developer.blender.org/D4942