[WIP] [Draft] Fast Pinch-Zoom-Pan support on the canvas sizer

This patch adds a new class ZoomHelper that
is used to control the zooming of the document
by API and/or touch gestures. CSS3D transforms
are used to do extremely fast bitmap scaling
of the sizer.
1-finger Panning is actually native scrolling so that you can see the scrollbars. Pinch-Zoom-Pan is smoother and non-native.



Not yet completely ready for code review, but it compiles fine.
1-finger panning is currently slightly broken and there is a silght flicker before and after a gesture's start and end, that is being worked upon. Pinch-zoom and panning during that works well.

**Just putting this up for anyone who wants to play with it. Will attempt to fix the remaining issues as fast as possible.**