Added outputCameras() function to RendererAlgo.h.

This allows all cameras, not just the primary camera, to be output to the renderer. Also added an overload for outputCamera() which outputs the camera from a specific location.
Made ExecutableRender output all cameras.

This allows secondary cameras to be used for stereo rendering, projections, dicing cameras etc.
Made InteractiveRender support multiple cameras.

Backwards incompatibility :

	- InteractiveRender "updateCamera" plug renamed to "updateCameras". I've done a search and can't find any reference to "updateCamera" in any IE code though.
Added support for per-camera resolution overrides.

These are specified with a "resolutionOverride" V2iData in the camera parameters. There's no way of generating such an override in Gaffer itself right now - this is just to support an internal Image Engine workflow for the moment.
Merge pull request #1060 from johnhaddon/multipleCameras

Multiple camera support