Fixed RendererAlgo outputScene() to include coordinate systems.
Added global mode to GafferScene::Attributes node.

This places the attributes in the globals (with "attribute:" prefixes) rather than at locations in the scene. This is for use in setting up "render globals" networks where in fact some of the values being specified are actually renderer attributes rather than options (maximum ray depth in 3delight for instance).
Updated SceneInspector to display global attributes.
Updated render nodes to support global attributes.
Fixed bug in Attributes node.

Also added some comments to sort things out so it's harder to get wrong in the future.
Added global attribute support to SceneProcedural.

Fixes #964.
Merge pull request #976 from johnhaddon/globalAttributes

Global attributes