For performance reasons render graphs can keep memory allocated so it
could be reused. This PR optimizes the memory usage inside the
rendergraph to keep it within normal usage.
I didn't detect any performance regression with this change but reduces
the memory when performing final image rendering of heavy scenes.
Partial fix for #137382. the amount of memory still increases with 4mb
per render. It fixes the main difference when using large scenes.
Pull Request: https://projects.blender.org/blender/blender/pulls/137660
13d6ba1f62