Add glowutils::Camera::projectionOffset

This adds the possibility to perform an additional translate on the vertices after the normal ModelViewProjectionMatrix was applied to them. This is needed, according to page 25 of the 0.2 version of the OculusSDK-documentation (http://static.oculusvr.com/sdk-downloads/documents/Oculus_SDK_Overview.pdf) to properly integrate the OculusRift.

I'm not sure whether this should be in the public interface of a general Camera, but I see no way to solve this via subclassing. If you do, or have an idea how I should change glowutils::Camera to make flexible manipulations to the matrices, please point me there.