Moved the responsibility for finding file to load on to the ReaderWriter plugins,

instead of osgDB::Registry where it original lay.  This has been done to allow
fileName strings to be encode data rather than just file names, such as one
requires when using PagedLOD along with plugins for doing dynamic tesselation.

src/osgDB/FileUtils.cpp

@@ -37,9 +37,6 @@
 
 bool osgDB::fileExists(const std::string& filename)
 {
-    // hack for getting TXP plugin to utilise PagedLOD.
-    if (getLowerCaseFileExtension(filename)=="txp") return true;
-
     return access( filename.c_str(), F_OK ) == 0;
 }