Render order preference

Initial work for render order based on ConstantMap.  Useful for 3D as 2D displays, or Globe type displays where an invented mapping to Display.ZAxis may not be desired.  If applications don't want to use this (they don't have to) everything will work as before.

Introduce new functionality to DisplayRenderer to allow dynamic scene reordering of DataRenderers - useful in the above situations

Significant testing in the HYDRA2 application with good results.  Eliminates unnecessary re-rendering of complex coastline data for example when add or removing images dynamically