Add pixelRatio to ol.source.Image#getImage
Add pixelRatio to ol.source.Image#createImage
Add pixelRatio param to ol.Image constructor
Use the image pixelRatio in ol.renderer.canvas.ImageLayer
Add hidpi option to ol.source.ImageWMS and ol.source.MapGuide
Add serverType option to ol.source.ImageWMS constructor
Merge pull request #1367 from fredj/vector-api-hdpi

[vector-api] High dpi devices support