Make options argument to ol.style.Style optional
Make options argument to ol.style.Fill optional
Make options argument to ol.style.Image optional
Make options argument to ol.style.Stroke optional
Make options argument to ol.style.Text optional
Add styleFunction property to ol.Feature
Add ol.layer.Vector.defaultStyleFunction
Use ol.layer.Vector.defaultStyleFunction by default
Don't attempt to render features whose style is undefined or null
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