Add pinch-zoom/drag of entire canvas

[GoogleCodeExporter]

If the user pinch-zooms or drags outside of an individual image in the photo 
sorter demo, the whole canvas should be zoomed/panned, not just individual 
photos.

This will require another coordinate system transform, and separate 
restrictions on what sorts of operations are valid on the canvas and on objects 
on the canvas (e.g. maybe the canvas can't be rotated, but objects can).

Original issue reported on code.google.com by luke.hutch on 30 Apr 2013 at 4:30

[masoudmm]

Hi there, first of all thank you for the app
I wonder how can I change your code to support multi touch simultaneously for several users, in other words users can zoom, rotate and drag several image simultaneously.
you are using the history of every touch event in your app, how can we use the specific history for every image object in multi touch canvas? is this a good idea??