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

source/blender/nodes/shader/node_shader_tree.c

@@ -844,7 +844,7 @@ void ntreeGPUMaterialNodes(bNodeTree *localtree, GPUMaterial *mat, bool *has_sur
 	ntree_shader_tag_nodes(localtree, output, &tags);
 
 	exec = ntreeShaderBeginExecTree(localtree);
-	ntreeExecGPUNodes(exec, mat, 1);
+	ntreeExecGPUNodes(exec, mat, output);
 	ntreeShaderEndExecTree(exec);
 
 	/* EEVEE: Find which material domain was used (volume, surface ...). */