Currently this is possible after built-in images are loaded in memory.
Allows to save memory used by dependency graph and copy-on-write.
In practice this lowers peak system memory usage from 52GB to 42GB on
a production file of spring 03_035_A.lighting.
Note, that this only applies to F12 and command line renders.
Bigger note, that this optimization is currently only possible if
there are no grease pencil objects to be rendered.
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