Cycles: support for custom shader AOVs

Custom render passes are added in the Shader AOVs panel in the view layer
settings, with a name and data type. In shader nodes, an AOV Output node
is then used to output either a value or color to the pass.

Arbitrary names can be used for these passes, as long as they don't conflict
with built-in passes that are enabled. The AOV Output node can be used in both
material and world shader nodes.

Implemented by Lukas, with tweaks by Brecht.

Differential Revision: https://developer.blender.org/D4837

source/blender/makesrna/intern/rna_nodetree.c

@@ -5187,6 +5187,19 @@ static void def_sh_tex_ies(StructRNA *srna)
   RNA_def_struct_sdna_from(srna, "bNode", NULL);
 }
 
+static void def_sh_output_aov(StructRNA *srna)
+{
+  PropertyRNA *prop;
+
+  RNA_def_struct_sdna_from(srna, "NodeShaderOutputAOV", "storage");
+
+  prop = RNA_def_property(srna, "name", PROP_STRING, PROP_NONE);
+  RNA_def_property_ui_text(prop, "Name", "Name of the AOV that this output writes to");
+  RNA_def_property_update(prop, NC_NODE | NA_EDITED, "rna_Node_update");
+
+  RNA_def_struct_sdna_from(srna, "bNode", NULL);
+}
+
 static void def_sh_script(StructRNA *srna)
 {
   PropertyRNA *prop;