Blender Git Loki

Small usability tweaks in Compositor:

- After making a Group, internal socket values are now copied to the
group node, so an execute will give identical results
- After ungrouping, the cyclic dependency tagging didn't work well in
all cases, showing a red noodle line and giving composite errors.
For time being solved by calling the NodeSort twice.
- Viewer nodes now get previews updated always (used to be only the active)
Note: this is not for previews inside of groups!