add devicePixelRatio support but only to renderer side
add devicePixelRatio support but only to renderer side

as suggested by https://github.com/cytoscape/cytoscape.js/pull/285

KonstantinShemyak/cytoscape.js,rlugojr/cytoscape.js,rlugojr/cytoscape.js,cytoscape/cytoscape.js,KonstantinShemyak/cytoscape.js,cytoscape/cytoscape.js,soomin-maeng/cytoscape.js,soomin-maeng/cytoscape.js