Add tilePixelRatio option to ol.source.Tile
Add tilePixelRatio option to ol.source.TileImage
Add tilePixelRatio option to ol.source.WMTS
Add example
Use tilePixelOption in wmts-hidpi example
Add docs for tilePixelRatio
Merge pull request #2216 from elemoine/pixelratio

Add tilePixelRatio option to ol.source.WMTS