The source determines who can do GetFeatureInfo

Now that the ol.renderer.Layer base class has a
getFeatureInfoForPixel method, we have to check whether the
source supports GetFeatureInfo, not the layer renderer.
Avoid source check in map renderer

By returning true or false from the layer renderer's
getFeatureInfoForPixel method, we know whether the source
supports GetFeatureInfo or not.
Merge pull request #1298 from ahocevar/gfi-callback

The source determines who can do GetFeatureInfo