Todo/feature request

When using masks or other simple 3D elements in composites, doing
a layer re-rendering on a node is a bit clumsy all the time.

This commit does two things to help:
- new hotkey "Z" in node editor automatically finds render layer
  that changed and re-renders it + composites
- option "Auto Render" does same, but then after every transform
  edit in 3D window

The latter is experimental; real & proper system for this requires
full threaded render support (like previews). But it works!

Demo file:
http://download.blender.org/demo/test/auto_composite.blend

Important fix:
After any render, all the render layers were tagged "changed", which
caused any edit to first totally recomposte everthing. Now it only
composites changes.

Implementation notes

- DAG scene flush now sets 'changed' flags in render layer nodes
- Added notifier for 'transform finished' to trigger the update,
  this is temporarily.

release/scripts/ui/space_node.py

@@ -71,6 +71,7 @@ class NODE_HT_header(bpy.types.Header):
             if snode.show_backdrop:
                 row = layout.row(align=True)
                 row.prop(snode, "backdrop_channels", text="", expand=True)
+            layout.prop(snode, "use_auto_render")
 
         layout.separator()