pinch gesture trackpad support

[rupertlevene]

Firefox on OS X supports pinch zooming pages, with the appropriate settings in about:config. But when used to zoom in on a pdf file, things get blurry (as they do, incidentally, when the keyboard shortcuts Apple + and Apple - are used).

The pinch zoom in/out gestures should be mapped to the same place as the + and - keys in the pdf.js UI.

[gigaherz]

This is the same issue as using control-[+]/[-] and control-mousewheel to zoom.

Ref: #902

[timvandermeij]

Closing as duplicate of #2582.