Blender Git Loki

Three-in-one commit:

- Compositor now is threaded
Enable it with the Scene buttons "Threads". This will handle over nodes to
individual threads to be calculated. However, if nodes depend on others
they have to wait. The current system only threads per entire node, not for
calculating results in parts.

I've reshuffled the node execution code to evaluate 'changed' events, and
prepare the entire tree to become simply parsed for open jobs with a call
to node = getExecutableNode()
By default, even without 'thread' option active, all node execution is
done within a separate thread.

Also fixed issues in yesterdays commit for 'event based' calculations, it
didn't do animated images, or execute (on rendering) the correct nodes
when you don't have Render-Result nodes included.

- Added generic Thread support in blenlib/ module
The renderer and the node system now both use same code for controlling the
threads. This has been moved to a new C file in blenlib/intern/threads.c.
Check this c file for an extensive doc and example how to use it.

The current implementation for Compositing allows unlimited amount of
threads. For rendering it is still tied to two threads, although it is
pretty easy to extend to 4 already. People with giant amounts of cpus can
poke me once for tests. :)

- Bugfix in creating group nodes
Group node definitions demand a clear separation of 'internal sockets' and
'external sockets'. The first are sockets being linked internally, the latter
are sockets exposed as sockets for the group itself.
When sockets were linked both internal and external, Blender crashed. It is
solved now by removing the external link(s).