Make forEachFeatureAtPixel work with layer groups
Merge pull request #1372 from elemoine/vector-api-layer-group

[vector-api] Fix forEachFeatureAtPixel bug
Merge pull request #1381 from twpayne/vector-api-feature-style-function

[vector-api] Per-feature styleFunction
Add devicePixelRatio option to ol.Map constructor
Add devicePixelRatio to ol.FrameState
Scale output canvas by devicePixelRatio
Pass pixel ratio to ol.render.canvas.ReplayGroup constructor
Pass pixel ratio to ol.render.canvas.Replay constructors
Scale line widths by pixel ratio in replay mode
Pass pixel ratio to ol.renderer.canvas.VectorLayer#renderFeature
Simplify geometries to a tolerance of half a device pixel
Pass pixel ratio to ol.render.canvas.Immediate constructor
Scale line widths by pixel ratio in immediate mode
Set zoom in vector-layer example to match master