Fixed bug in IECoreRI::Renderer camera editing.

It was incorrectly storing m_lastCamera (the primary camera) even during edits, meaning that if cameras other than the primary camera were edited, it would lose track of the primary camera. For common sequences of edits (all cameras in order, primary last), this meant the main world camera would never be updated.
Merge pull request #351 from johnhaddon/multipleCameraEditsFix

Fixed bug in IECoreRI::Renderer camera editing.