[librender] correct management of node metadata (e.g. position given by the editor)

...R/Gui/NodeEditor/include/nodes/internal/NodeDataModel.hpp

src/libRender/RadiumNBR/Gui/NodeEditor/src/FlowScene.cpp

...nder/RadiumNBR/Gui/RenderGraphEditor/NodeAdapterModel.hpp

...RadiumNBR/Gui/RenderGraphEditor/RenderGraphEditorView.cpp

src/libRender/RadiumNBR/NodeGraph/Node.cpp

src/libRender/RadiumNBR/NodeGraph/Node.hpp

...sources/RadiumNBR/RenderGraphs/RadiumForwardRenderer.flow