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/editors/space_node/drawnode.c

@@ -1154,6 +1154,11 @@ static void node_shader_buts_white_noise(uiLayout *layout, bContext *UNUSED(C),
   uiItemR(layout, ptr, "noise_dimensions", 0, "", ICON_NONE);
 }
 
+static void node_shader_buts_output_aov(uiLayout *layout, bContext *UNUSED(C), PointerRNA *ptr)
+{
+  uiItemR(layout, ptr, "name", 0, NULL, ICON_NONE);
+}
+
 /* only once called */
 static void node_shader_set_butfunc(bNodeType *ntype)
 {
@@ -1310,6 +1315,9 @@ static void node_shader_set_butfunc(bNodeType *ntype)
     case SH_NODE_TEX_WHITE_NOISE:
       ntype->draw_buttons = node_shader_buts_white_noise;
       break;
+    case SH_NODE_OUTPUT_AOV:
+      ntype->draw_buttons = node_shader_buts_output_aov;
+      break;
   }
 }