[WIP] Raster reprojection for 2.x.x

[samuel-girard]

Hi,

Following #37, I created a new branch (same as #562 and #581 but rebased on master because many things changed from 1.x.x to 2.x.x).
I still have the issues described in https://github.com/openlayers/ol-cesium/issues/37#issuecomment-371204358 and https://github.com/openlayers/ol-cesium/issues/37#issuecomment-370206118, but it happens only in the example projects for me. I have been successfully using ol-cesium in my own project with these changes included in the last 6 months with no issue yet found.

I don't know where the problem lies, but it would be so nice to see this feature (image layers reprojection) included in ol-cesium, so if anyone has an idea...