Update D3 example with improved version

[Jonny Burger]

From message posted on Discord by d3ch0

Ok, here is the updated version, cleaned up and added a couple of comments. When rendered, both the original and refactored version of the video output a file with the exact same sha256 sum:

a4051dbc731baebcd896b6ec6182750012dfbe5e14ad6ddd99f2c6220363b02b

https://gist.github.com/virtuallyunknown/27d19f38dccb9e19245a9bcd2a95f447

Cheers!