Integrate image view transitions, when creating the graph

[Sylvain Doremus]

Is your feature request related to a problem? Please describe.
In order to be able to generate render pass creation info, I need to have the image view transitions explicited in the graph.

Describe the solution you'd like
Just create a wrapper structure (let's say crg::AttachmentTransition), with two crg::Attachment inside it.
A crg::AttachmentTransitionArray would replace the current crg::AttachmentArray in crg::GraphNode.