Try to infer screen densities from viewport widths

[nhoizey]

As most Analytics solutions only provide viewport widths and nothing about screen densities, this would help use Daltons without requiring addition of custom dimensions months in advance.

For example, we know that there is a strong correlation between thinner viewports and high density:
https://twitter.com/TimVereecke/status/1121757778400481282

[nhoizey]

Guesses would be much better with screen dimensions instead of viewport widths, especially for desktops where windows are much less used full screen than on mobiles.