Expose map renderer type in the API

This pull request relates to issue #2004
Indeed @tschaub, after reading your input, it's seems to me that it should be the subject of a specific PR.

About `goog.functions.constant`, I am far from a Google Closure expert but according to the [source code](http://docs.closure-library.googlecode.com/git/local_closure_goog_functions_functions.js.source.html), it just return a function...

For the sake of argument, I find rather bad the idea of Google Closure to "fake" the constant concept  in a language that does not have such concept in the first place.