Merge remote-tracking branch 'upstream/master' into vector-api

Conflicts:
	src/objectliterals.jsdoc
	src/ol/attribution.js
	src/ol/geom/geometry.js
	src/ol/geom/geometrycollection.js
	src/ol/geom/linestring.js
	src/ol/layer/vectorlayer.exports
	src/ol/layer/vectorlayer.js
	src/ol/map.js
	src/ol/proj/proj.js
	src/ol/renderer/canvas/canvasvectorlayerrenderer.js
	src/ol/source/imagewmssource.js
	src/ol/source/tilewmssource.js
	src/ol/source/vectorsource.exports
	src/ol/source/vectorsource.js
	src/ol/source/wmssource.js
	src/ol/style/style.js
	src/ol/tilegrid/tilegrid.js
	src/ol/tilegrid/wmtstilegrid.js
	src/ol/tilegrid/xyztilegrid.js
Use olx options ns in icon and geojson examples
Add devicePixelRatio option to ol.Map constructor
Add devicePixelRatio to ol.FrameState
Add pixelRatio to ol.source.Image#getImage
Add hidpi option to ol.source.ImageWMS constructor
Scale output canvas by devicePixelRatio
snapshot