Expose the map renderer type to the public API

In the case  where the map decides itself which renderer to use. It can
be convenient for the developer to have knowledge of the choice made.
Rename ol.RendererHint to ol.RendererType
Provide ol.RendererType structure through ol.renderer.Map

It was previously in ol.Map, but it could not be provided because
of circular dependencies.
Rename Map.DEFAULT_RENDER_HINTS to Map.DEFAULT_RENDERER_TYPES
Merge pull request #2033 from htulipe/map-renderer-type

Expose map renderer type in the API.