Support pinch-to-zoom with Mac trackpads on desktop Safari

[maxkfranz]

Issue type

Feature request

Description of new feature

Pinch-to-zoom should work the same as it does on Chrome for Mac, if you use a supported trackpad like Apple's Magic Trackpad.

Motivation for new feature

The library already supports pinch-to-zoom on desktop in Chrome and Firefox. Those browsers emit scroll events for pinch gestures. Safari supports gesture* events for the same pinch gesture. So it would suffice to use those gesture* events with the same zoom handler that Chrome and Firefox use.