Camera additions

This adds pixel aspect ratio, resolution multiplier and overscan options to GafferScene. It requires https://github.com/ImageEngine/cortex/pull/335 for the pixel aspect ratio to work correctly, but there's no hard dependency - it's possible to build still with the existing cortex master.

Note that this doesn't add pixel aspect ratio support to the GafferImage side of things, just GafferScene (the Display node won't show the correct pixel aspect).