This was a mistake in the original code from D1079.
With the current way how direction ot mirror ball works camera should look
into negative Y direction. Corrected it in the camera matrix synchronization,
so no extra calculations are needed at the render time.
That's a bit annoying, but we'd better port it to the release branch, or
otherwise we'll end up with files created with wrong camera mapping after
2.75 release.
211a95b538