Add utility function to save an attachment of the webgpu render window to VTI

- This is a convenient function to write the attachment to a VTI file which can be later opened in ParaView for inspecting individual pixel values.
- Supported outputs are color (rgb/rgba), depth and ids.
- The `vti` file for Ids attachment will have a 4-component scalar array describing the cell id, prop id, composite id and process id.