Until we know what we want to return, return undefined
Interface for GetFeatureInfo

When a source implements this interface, the layer renderer can
access the source to get feature info.
Implementing GetFeatureInfo for WMS
Add example, and use getFeatures in other examples

The new dedicated getfeatureinfo example shows how to combine
feature info from a WMS and a vector layer. The other examples
that previously used getFeatureInfo from vector layers now use
the more appropriate getFeatures.
Adding tests, and fixing an issue revealed by the tests
No array needed for a single event

Thanks @bartvde.
Use X, Y or I, J depending on WMS version

This also adds consistency - Pixel coordinates are now always
rounded to integers.
Fixing example title

Thanks @bartvde for catching this.
Merge pull request #885 from ahocevar/wms-getfeatureinfo

Implement WMS GetFeatureInfo