Separate "desktop" and "mobile" zoom calculations.

This ensures that the layout viewport responds to each type of zoom correctly,
and lays the ground for CSS Media Queries and CSS Device Adaption.

Until we have proper touch support, mobile-style "pinch" zoom can be simulated
by holding Ctrl while scrolling with a mousewheel or trackpad gesture.
Merge pull request #2666 from mbrubeck/pinch

Separate "desktop" and "mobile" zoom calculations.