Load 640p webp poster image by default

[paulirish]

Currently lite-youtube loads the 480p jpeg by default, as that image is defined 100% of the time.
It then tries to tries to load the 640p webp, and switches to that if it exists.

see https://github.com/paulirish/lite-youtube-embed/blob/master/youtube-thumbnail-urls.md

However, the 640p webp is both higher resolution, and uses less bytes. (in almost all the cases i've found)

So.. we should probably use that as default background-image. We can do a img.src load test (that races) and if it fails, swap the bg image back to the 480jpeg.

This approach would make the most sense for videos published in the last ~4 years, and also optimizes for bytes.