On the fly raster reprojection for WMS sources

Add the capability to handle layers from WMS servers, event if theses layers are not available in the projection of the View.
This is an approximative raster reprojection using the WMS server capabilities, good enough at high zoom levels.

For exemple, with these modifications, it is possible to have a WMS layer only available in the EPSG:4326 projection inside of a Global Mercator view.
you can use the new ol.proj.transformExtent here - simpler