Compositor: File Output

File output working for stereo output
(so each individual input can be outputted as a separated stereo format)

Not supported at the moment:
* Multilayer + stereo output

Not fully working (though it should):
* Multiview
Compositor: File Output Multiview

Multiview now also working when used for the individual inputs. Multiview with multiple layers was already working.
Only remaining bit to work is Multilayer + StereoOutput
Disabling Stereo Output for Multilayer

This is doable, but again it is a lot of work for something that probably no one will use.
I do not know why I even coded multlayer+stereo output for regular rendering and image editor :|
I will see, maybe I implement that later, but just do not want to bother with this now.
safe tmp
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