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