Revert "Merge pull request #2609 from brson/parallel-render"

This reverts commit 850bd2891de589b95e32dc8f0b59d4043ed1e0a3, reversing
changes made to 5b0feac32a3ef13a76670eb2f5f21e7b598fc890.
Merge pull request #2685 from larsbergstrom/revert_2609

Revert "Merge pull request #2609 from brson/parallel-render"
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.