Giving the map a getFeatureInfoForPixel method

This method is an entry point for getting feature information.
Renderers can use a hit canvas or defer to a layer (source) to
get matching features for a pixel.

For now this is only implemented for vector layers, and it uses
a bbox query because we cannot refine the result because of
missing geometry intersection functions yet.
Get hit candidates from RTree, then refine result

Now we get exact hits also for lines and polygons.
Info box instead of tooltip as suggested by @fredj
Do not check for listeners, as suggested by @fredj
Merge pull request #674 from ahocevar/getfeatureinfo

Getting feature information for vector layers. r=@twpayne,@fredj