Expose rendering context in API

AFAIK, there is no method/member in the current API that can tell which rendering context is used by the map. This information could be useful.

For instance, some times ago, default renderer has been changed from WebGL to Canvas. This change caused a malfunction in [bind inputs example](http://ol3js.org/en/master/examples/bind-input.html) where hue, saturation and brightness inputs are no longer relevant in Canvas context. This results in a buggy-like example. One could easily fix that by disabling those output unless WebGL rendering is used.

IMHO, this case illustrates the need to make rendering information available to developers.

I would see something like: `map.renderingContext` which would be a string (`webgl`, `canvas` or `dom`).

If this make sense, I will make a PR.