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
96de11c2c6