Expose debug type into the interface

This way it is now possible to select which exact debug pass is to be used
by the render engine. Accessible from the Passes panel.

Currently it could only be one debug pass, in the future we can make menus
and image users smarter and support multiple passes of the same type.

intern/cycles/blender/addon/ui.py

@@ -431,6 +431,9 @@ class CyclesRender_PT_layer_passes(CyclesButtonsPanel, Panel):
         col.prop(rl, "use_pass_emit", text="Emission")
         col.prop(rl, "use_pass_environment")
 
+        if hasattr(rd, "debug_pass_type"):
+            layout.prop(rd, "debug_pass_type")
+
 
 class CyclesRender_PT_views(CyclesButtonsPanel, Panel):
     bl_label = "Views"