Basic implementation of RenderPass based architecture

[mvaligursky]

This is a first step in the implementation of #4271. ForwardRenderer, instead of directly rendering the scene, performs two steps:

1. analyses the scene, and constructs a FrameGraph of rendering passes. Only layers rendered by the cameras are fully split into render passes, other rendering systems (shadows, grab passes, post-effects and similar) are inserted into the frame graph as black boxes. Those will be further broken down into actual render passes in the future.
2. renders the generated render passes

Other changes:

• making Debug.setTrace public, to allow external applications to enable log tracing
• added few trace ids for debug logging related to render passes

example of the trace output for post-effects engine example

and here are the frame passes from the same example when postprocessing is disabled - this results in a smaller number of render passes, which in the future PRs will translate to a smaller number of multi-sampled resolves: