Add ol.geom.Polygon#getLinearRingCount function
Document ol.geom.Polygon#getLinearRing function
Use the feature style if defined

Same behavior as ol.layer.Vector
Use a custom style for the position feature
Merge pull request #2676 from fredj/featureoverlay-style

ol.FeatureOverlay: use the feature style if defined
Add TextStyle offsets getters to API.

The other properties getters are in the API already.
Merge pull request #2677 from gberaudo/export_text_style_offsets

Add TextStyle offsets getters to API.
Merge pull request #2534 from fredj/polygon-api

Document ol.geom.Polygon#getLinearRing function
Make tileSize a config option for ol.source.XYZ
Merge pull request #2681 from ahocevar/xyz-tilesize

Make tileSize a config option for ol.source.XYZ
Unset z-index in overlay example
Merge pull request #2692 from fredj/overlay-index

Unset z-index in overlay example
Remove undefined from ol.Map#getLayers return type

It's already assumed that the collection is defined in ol.Map#addLayer
and ol.Map#removeLayer.
Disallow undefined layers collection
Change signature of ol.layer.Group#getLayers

This commit changes the signature of ol.layer.Group#getLayers from {ol.Collection.<ol.layer.Base>|undefined} to {!ol.Collection.<ol.layer.Base>}. In this way the caller is guaranteed that getLayers returns a dereferencable object.