Added osgDB::readShaderFileWithFallback(..) convinience functions to make it easier to set up reading external shader files with a built in fallback.


git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@14636 16af8721-9629-0410-8352-f15c8da7e697
Aded shaders for various combinations of layers.


git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@14637 16af8721-9629-0410-8352-f15c8da7e697