Added .tf & .tf-255 plugin for reading ascii 1D transfer functon files in support for volume rendering.

git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@14448 16af8721-9629-0410-8352-f15c8da7e697

src/osgDB/Registry.cpp

@@ -424,6 +424,9 @@ Registry::Registry()
     // addFileExtensionAlias("pfa",   "freetype");  // type2 ascii
 
 
+    // TransferFunction head
+    addFileExtensionAlias("tf-255", "tf");
+
     // portable bitmap, greyscale and colour/pixmap image formats
     addFileExtensionAlias("pbm", "pnm");
     addFileExtensionAlias("pgm", "pnm");