Easy access to a canvas scale factor needed.

[wojtekzozlak]

I wanted to scale labels font-size during zooming, but I couldn't find any way to get a Canvas scale factor in documentation. I've ended up with using a Canvas.scaleOffsetX property, but it looks kinda ugly. There should be a simple way to obtain zooming factor.

[DanFelton]

canvas.scaleOffsetX canvas.scaleOffsetY