Blender Git Loki

Bake-API: Test for cyclic node connection

If the active image node contributes to the final material shader
(meaning it's either directly or indirectly connected to an Output Node)
the user will receive an alert about circular dependency.

Similar to what we do for Blender internal the baking will still happen,
but the user will receive the alert which should prevent the image
saving to happen if the result was not intentional.

Core function to check for node output written by Lukas Toenne.

Reviewers: lukastoenne, campbellbarton

Differential Revision: https://developer.blender.org/D673