Multiview Builtin Stereo camera support

@brechtvl ,

I will still go over the issues you raised later this week (so many, thanks a lot for the review so far).

Meanwhile I started the builtin camera code.
Any thoughts on whether you think this is the right direction?

It works for Cycles, BI and Viewport (calculation is a placeholder for now).

Also, I get a few Memoryblock free: attempt to free illegal pointer once quitting Blender, so I'm suspecting there is something strange going on ...

Thanks