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

Rendering/WebGPU/vtkWebGPUPolyDataMapper.h

@@ -442,8 +442,6 @@ protected:
     int LastRepresentation = VTK_SURFACE;
     bool LastHasRenderingTranslucentGeometry = false;
   };
-  std::map<std::pair<vtkWeakPointer<vtkActor>, vtkWeakPointer<vtkRenderer>>, ActorState>
-    CachedActorRendererProperties;
 
 private:
   friend class vtkWebGPUComputeRenderBuffer;
@@ -488,6 +486,8 @@ private:
     CellDataAttributes::CELL_COLORS, CellDataAttributes::CELL_NORMALS
   };
 
+  std::map<std::pair<vtkActor*, vtkRenderer*>, ActorState> CachedActorRendererProperties;
+
   vtkWebGPUPolyDataMapper(const vtkWebGPUPolyDataMapper&) = delete;
   void operator=(const vtkWebGPUPolyDataMapper&) = delete;
 };