Use a simplified form of /get_scene that avoids the need
for a second service call.

The Scene3D plugin from ign-gui2 will make a service
request to get information about a scene, with the
service name configurable via xml. It expects
different data types than delphyne though, so
some more changes are needed.

ignition-rendering2 did not incoroporate the culling to materials
and it is causing Scene3D plugin not to properly decode the URI
and convert it into a file path.