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Python API: option for render engines to disable image file saving

For some custom rendering engines it's advantageous not to write the image files to disk.
An example would be a network rendering engine which does it's own image writing.

This feature is only supported when bl_use_postprocess is also disabled, since render
engines can't influence the saving behavior of the sequencer or compositor.

Differential Revision: https://developer.blender.org/D11512