Improve node collection and frame prepHEADdev

Improves the collection of nodes and rendering data during the preparation
phase by:

- Only traversing and collecting nodes when the node tree changes.
- Assigning a unique ID to each node in the tree for fast access to its
  associated rendering data.
- Having a single root node for the entire window. If there are multiple
  views within a window, they all operate on their respective sections
  of the tree.
- Storing rendering data separately which avoids collecting and copying
  in addition to make it more feasibly to just uploading the data in bulk
  when needed.
- Proper separation of data allows for the calculation of non-dependent
  data in parallel, once the global state has been updated.

This is merely the initial step towards a more advanced storage solution.
However, some adjustments are still necessary to transition a significant
portion of the preparation phase to the GPU. Our somewhat contrived
benchmark demonstrates a 6x improvement, which is also applicable to the
worst case scenario when nodes are dynamically added.

Change-Id: I1dd12734f6a043ca8145cb1c667921bb664aa3db
Reviewed-by: Christian Strømme <[email protected]>

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