Rendering - Move PipelineStack to Pipeline Compositor

[tadgem]

Need to rework this to use a more flexible structure, a graph seems more appropriate. This will involve a complete rewrite of the stack as it exists now but other types should be reusable.

Change will consist of:

• PipelineStack- PipelineGraph rewrite.
• Serialization of Graph
• ImGui editor for Graph.

[tadgem]

Need to rework this to use a more flexible structure, a compositor using a graph representation seems more appropriate. This will involve a complete rewrite of the stack as it exists now but other types should be reusable.

Change will consist of:

PipelineStack- Pipeline Compositor rewrite.
Serialization of Compositor
Easy user facing API
Expose to Lua
Expose to graph script when we get back round to that (not in scope of this issue)