Currently we lookup the `devicePixelRatio`, with fallback handling, in a number of spots in the code-base.
Rather than duplicating code we can instead add a new static method in the `OutputScale` class, since that one is now exposed in the API.
It lets the user make a pinch gesture with a finger on page with a drawing
and the second finger on an other page.
On mobile, it's pretty easy to be in such a situation.
