ol.layer.Vector.getSource return type specialisation.
Remove unnecessary type cast
Merge pull request #2651 from gberaudo/layer_vector_source_specialisation

ol.layer.Vector.getSource return type specialisation.
Add olx.layer.ImageOptions typedef
Set ol.layer.Image#getSource return type to ol.source.Image
Remove unnecessary type cast
Set ol.layer.Tile#getSource return type to ol.source.Tile
Remove unnecessary type cast
Don't expose olx.layer.LayerOptions
Don't expose olx.layer.BaseOptions