make render handling for > 1 image port more flexible
make render handling for > 1 image port more flexible

While Pipeline only supports a maximum of 2 input image nodes, branchForNode will be able to construct trees with more complexity and the rendering loop can as well.

renderStackForRootBranch will need updates to support > 2.

pizthewiz/Cinder-Pipeline,pizthewiz/Cinder-Pipeline