Handle more than one declared display

[bentoogood]

Currently only the last declared display will generate an output, as all displays are declared as

Display "/path/to/output"

where as displays other than the primary one (rgba) should be

Display "+/path/to/output"

[johnhaddon]

This is a bug in IECoreRI::Renderer, but is being tracked here as Gaffer is the place where it manifests itself to the user.

[johnhaddon]

I've got a pull request for Cortex which addresses this problem :

ImageEngine/cortex#112

[johnhaddon]

Pull request made it into Cortex 8.0.0-a18 - closing.