This patch replaces the Composite node with the Group Output node as the
primary compositor output. The old node was removed and versioned. This
was done for consistency with Geometry Nodes and in preparation for more
generic use of the compositor in VSE modifiers, layered compositing, NPR
multi-stage compositing, and more.
The Group Output node relies on the node tree interface, so we now have
a default interface of a single input and a single output. For now, only
the first input is considered while the rest are ignored, just like the
Geometry Nodes design. Furthermore, the input is required to be of type
color. Warnings and errors are issues if any of those are not met, also
similar to Geometry Nodes.
This introduces a new limitation: Composite outputs can no longer exist
in node groups, since they obviously then act as their respective group
outputs.
A refactor for the compositor scheduler is needed to simplify the logic
after this change, but this will be done in a separate patch.
Pull Request: https://projects.blender.org/blender/blender/pulls/142232
e34e6373b6