Added lens correction support for the Oculus Rift and did some minor cleanups.

* Added lens correction support for the Oculus Rift (the whole scene gets rendered to a new fullscreen RT where the lens correction is applied in two passes - one for each eye) - without Oculus support everything stays the same
* Renamed RenderType to StereoRenderState
* Removed underscores from the members of the base Camera class