Blender Git Loki

A couple of render improvements;

- Bug fix: the upper tile in a collumn for Panorama render didn't put the
mainthread to sleep properly. Now panorama renders 25% faster if you had
set Y-Parts to 4.

- Enabling Compositing in Scene for first time now adds a "Composite" node
too, so render output gets applied.
- An attempt to render with "Do Composite" without "Composite" node will
throw an error and stops rendering. In background mode it will just not
render at all, and print errors.
- Errors that prevent rendering now give a popup menu again.
- Having MBlur or Fields option on will now normally render, but with an
error print in console (not done yet...)