Add new devicePixelRatio map option
Add devicePixelRatio property to ol.FrameState
Add devicePixelRatio param to ol.source.Image#getImage
Scale the map canvas layer by frameState.devicePixelRatio
snapshot

todo:
 - fix extent
 - fix event coordinates
 - scale vector line width and text size
 - scale simplify tolerance
 - add 'retina' (boolean) property to source (wms)
 - fix frameState.coordinateToPixelMatrix