The DatabasePager now passes the Terrain pointer into the ReaderWriter's via osgDB::Options object,
rather than pushing a NodePath containing the Terrain onto NodeVisitor. This
change means that the DatabasePager nolonger needs to observer the whole NodePath and
will be lighter and quicker for it.
The change also means that ReadFileCallback can now run custom NodeVisitor's on the scene graph without
having to worry about TerrainTile's constructing scene graphs prior to the Terrain being assigned.
Also changed is the NodeVisitor::DatabaseRequestHandler which now requires a NodePath to the node that you wish
to add to rather than just the pointer to the node you wish to add to. This is more robust when handling scenes
with multiple parental paths, whereas previously errors could have occurred due to the default of picking the first
available parental path. This change means that subclasses of DatabasePager will need to be updated to use this new
function entry point.
Introduced a new callback osgDB::FindFileCallback that overrides the default behavior of findDataFile/findLibraryFile.
Introduced support for assigning ReaderWriter::Options directory to PagedLOD.
Introduced new osgDB::FileLocationCallback for assistancing the DatabasePager to know when a file is hosted on a local or remote file system.
Introduced a new FindFileCallback to Registry to compliement the existing ReadFileCallback and WriteFileCallback.
Added support for assign Find, Read and WriteFileCallbacks to osdDB::Options to enable plugins/applications to override the callbacks just for that
read/write call and any nested file operations
