Early Ehlinger reported a deadlock when a script tells Blender to render an animation and there are pynodes. While investigating I saw related crashes in bg (blender -b) mode, still not fixed. This commit tries to fix the problem for interactive mode, then. What it does is releasing the lock before rendering and relocking after that, like theeth suggests in the bug report: http://projects.blender.org/tracker/?func=detail&atid=125&aid=17389&group_id=9