Integration with existing wgpu projects

[hecrj]

This PR enhances the flexibility of iced_wgpu by allowing a Renderer to draw to any given TextureView while reusing an existing Device and CommandEncoder. Therefore, integrating iced with existing wgpu projects is now possible! 🎉

I have created an example (integration) which showcases how this can be achieved with iced_winit and iced_wgpu. It is important to note that the process should be the same with a different shell and/or renderer:

1. Initialize data structures
2. Map shell events to iced events
3. Process iced events
4. Update the user interface
5. Draw the user interface

This process, although very flexible, is currently not as straightforward as I would like it to be. However, I prefer to wait, see how folks implement it and gather more use cases before designing further abstractions.

[parasyte]

This might actually mostly fit my requirements! I'll give it a try. :)