Moved Registry::ReadFileCallback + WriteFileCallback, and osgDB::ReaderWriter::Options into their own separate Options file and into the osgDB namespace.

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.

src/osgDB/Options.cpp

@@ -108,4 +108,6 @@ Options::Options(const Options& options,const osg::CopyOp& copyop):
     _pluginStringData(options._pluginStringData),
     _findFileCallback(options._findFileCallback),
     _readFileCallback(options._readFileCallback),
-    _writeFileCallback(options._writeFileCallback) {}
+    _writeFileCallback(options._writeFileCallback),
+    _fileLocationCallback(options._fileLocationCallback),
+    _fileCache(options._fileCache) {}