Expose map renderer type in the API

This pull request relates to issue #2004
I *think* the only benefit of this is to save a few bytes (I don't think it has any runtime benefit).  If we want to be consistent in our use of `goog.functions.constant`, my suggestion would be to handle this separately (and also do it for `ol.format.BinaryFeature#getType`, `ol.format.JSONFeature#getType`, `ol.format.TextFeature#getType`, `ol.format.XMLFeature#getType`, `ol.geom.Circle#getType`, `ol.geom.GeometryCollection#getType`, `ol.geom.LinearRing#getType`, `ol.geom.LineString#getType`, `ol.geom.MultiLineString#getType`, `ol.geom.MultiPoint#getType`, `ol.geom.MultiPolygon#getType`, `ol.geom.Point#getType`, `ol.geom.Polygon#getType`, `ol.webgl.shader.Fragment#getType`, `ol.webgl.shader.Vertex#getType`, and probably more (that's a lot of bytes!)).