Fix T37713: proxy not updated for first frame in background render.

The problem is that the animation curves are first before object updates,
where BKE_pose_rebuild and proxy synchronize will undo the animation when the
proxy is evaluted for the first time after loading.

This is somewhat of a workaround but it's also useful to have the scene updated
once when opening in background mode, and it makes rendering from the UI and
background more similar and so more predictable to give the same result.
Merge branch 'master' of http://git.blender.org/blender