add retina scaling effect to dataUrl exporter
add retina scaling effect to dataUrl exporter

Add retina scaling as a normal dataurl multiplier.
Removed setWidth and setHeight in favor of setDimensions
Removed rescaling back of canvas because setDimension will clear the transform anyway.
kuldipem/fabric.js,rockmacaca/fabric.js,kuldipem/fabric.js,rockmacaca/fabric.js