Render: make Cycles and Evee support each other's output material nodes.

This changes the Cycles exporting and Cycles/Eevee UI code to support both
output material nodes, giving priority to the renderer native one. Still
missing is Eevee code to prefer the Eevee output node.

source/blender/nodes/shader/node_shader_tree.c

@@ -214,6 +214,9 @@ static void ntree_shader_link_builtin_normal(bNodeTree *ntree,
  * render engines works but it's how the GPU shader compilation works. This we
  * can change in the future and make it a generic function, but for now it stays
  * private here.
+ *
+ * It also does not yet take into account render engine specific output nodes,
+ * it should give priority to e.g. the Eevee material output node for Eevee.
  */
 static bNode *ntree_shader_output_node(bNodeTree *ntree)
 {

source/blender/nodes/shader/nodes/node_shader_output_material.c

@@ -42,10 +42,6 @@ static int node_shader_gpu_output_material(GPUMaterial *mat, bNode *node, bNodeE
 {
 	GPUNodeLink *outlink;
 
-	if (BKE_scene_uses_blender_eevee(GPU_material_scene(mat))) {
-		return false;
-	}
-
 	GPU_stack_link(mat, node, "node_output_material", in, out, &outlink);
 	GPU_material_output_link(mat, outlink);