Blender Git Loki

Fix T62434: EEVEE not using correct World Output node

We were already getting the designated output node in
'ntreeGPUMaterialNodes()' but this wasnt used in 'ntreeExecGPUNodes()',
instead whatever node was tagged NODE_DO_OUTPUT was executed.

note: this is just the bare minimum to fix the bug, other improvements
previously done in D4482 might follow as a separate commit.

Reviewers: brecht, fclem

Maniphest Tasks: T62434

Differential Revision: https://developer.blender.org/D4630