There are very rare use-cases for having multiple Group Output nodes. However,
it's something that's been supported for a long time and removing it may be a
regression for some. In practice, it's usually a mistake when someone has
multiple Group Output nodes.
This patch adds a simple warning on inactive Group Output nodes to help the user
notice this case.
Pull Request: https://projects.blender.org/blender/blender/pulls/138743
dd47ee9e25