Added various improvements for the Oculus VR integration.

* The scene is now rendered to render targets which can be larger than the actual size provided by the Display class - this is currently used to compensate for the lens correction algorithm which needs the final result to be upscaled afterwards. To avoid a loss of quality (and an even more pixelated image in the Rift), the render targets are scaled by the scale factor used for the lense correction algorithm beforehand and get downsampled in the end to the actual display width and height.
* Fixed that rendering the scene multiple times for each eye caused the render queues and the thus the triggered chunk updates to become a bit confused - optimized this while I was at it (don't fill the shadow render queue multiple times, ...).
* Disabled motion blur when Rift support is activated - dear lord... that nearly made me puke on my desk