Use the absolute value of the scale when calculating margins

Fixes gh-77
Release 1.12.4
Performance: promote the panzoom element to 3D using backface-visibility

- Setting backface-visibility gives the element it's own compositor layer where supported
innerWidth -> outerWidth
Release 1.12.5
Merge branch 'master' into gh-pages

* master:
  Release 1.12.5
  innerWidth -> outerWidth
  Performance: promote the panzoom element to 3D using backface-visibility
  Release 1.12.4
  Use the absolute value of the scale when calculating margins