Space_node: Add draw backdrop callback.

Add a callback to allow custom node editors to draw their own
backdrop.

Differential Revision: https://developer.blender.org/D4709

Reviewed by: JacquesLucke

source/blender/python/intern/bpy_rna_callback.c

@@ -54,6 +54,7 @@ static const EnumPropertyItem region_draw_mode_items[] = {
     {REGION_DRAW_POST_PIXEL, "POST_PIXEL", 0, "Post Pixel", ""},
     {REGION_DRAW_POST_VIEW, "POST_VIEW", 0, "Post View", ""},
     {REGION_DRAW_PRE_VIEW, "PRE_VIEW", 0, "Pre View", ""},
+    {REGION_DRAW_BACKDROP, "BACKDROP", 0, "Backdrop", ""},
     {0, NULL, 0, NULL, NULL},
 };