webgpu: use raw pointer instead of vtkWeakPointer

- This commit includes an optimization for large number of blocks or actors
  O(10,000) and higher.
- It gives a 10x performance boost. Previously, rendering 10,000 actors
  or 10,000 blocks in one actor with composite mapper took ~200 milliseconds
  per frame.
- With this commit, it only takes 20-25 ms per frame.
- The mapper's internal cache of actors is queried in every `RenderPiece`.
  During the query, when constructing a key for the `map::find`,
  the constructor and destructor of the vtkWeakPointer from a raw
  pointer took up roughly 50-60ms when repeated over O(10,000) actors