Update `wgpu` and `winit` versions in the Wgpu example

[tgross35]

These dependencies were quite out of date and were hitting some bugs when running. Update dependencies to the latest and do needed restructuring to make the example run.

Also use the Winit changes as a reason to break the code into a more logical structure, and add a gif to show what the demo does (I had no clue!).

Part of #884

[tgross35]

This doesn't completely fix #884 since I wasn't able to get to the SVG example. Still that is pretty significant, I am glad to be able to actually run the demo now :)

[nical]

Thanks!