[css-device-adapt] arbitrary resolutions

The resolution descriptor in @viewport takes 3 values:

auto
device
<resolution>

In this context, resolution currently defines dpi to mean "css pixels per  
physical inch", which is the exact opposite of what dpi means everywhere  
else. I first though of
fixing it to mean the same thing as elsewhere (physical dots per css  
inch), but upon further reflexion, I think we should just remove the  
ability to specify a <resolution> here.

There are good use cases for auto and device, but I am quite convinced  
that using explicit numbers here is always a misguided thing.

Therefore, I propose that we just allow 2 values (auto and device), and  
drop <resolution>.

  - Florian