Introduced CMake option OSG_PROVIDE_READFILE option that defaults to ON, but when switched to OFF disables the building of the osgDB::read*File() methods,

forcing users to use osgDB::readRef*File() methods.  The later is preferable as it closes a potential threading bug when using paging databases in conjunction
with the osgDB::Registry Object Cache.  This threading bug occurs when one thread gets an object from the Cache via an osgDB::read*File() call where only
a pointer to the object is passed back, so taking a reference to the object is delayed till it gets reassigned to a ref_ptr<>, but at the same time another
thread calls a flush of the Object Cache deleting this object as it's referenceCount is now zero.  Using osgDB::readREf*File() makes sure the a ref_ptr<> is
passed back and the referenceCount never goes to zero.

To ensure the OSG builds when OSG_PROVIDE_READFILE is to OFF the many cases of osgDB::read*File() usage had to be replaced with a ref_ptr<> osgDB::readRef*File()
usage.  The avoid this change causing lots of other client code to be rewritten to handle the use of ref_ptr<> in place of C pointer I introduced a serious of
templte methods in various class to adapt ref_ptr<> to the underly C pointer to be passed to old OSG API's, example of this is found in include/osg/Group:

    bool addChild(Node* child); // old method which can only be used with a Node*

    tempalte<class T> bool addChild(const osg::ref_ptr<T>& child) { return addChild(child.get()); } // adapter template method

These changes together cover 149 modified files, so it's a large submission. This extent of changes are warrent to make use of the Object Cache
and multi-threaded loaded more robust.



git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/branches/OpenSceneGraph-3.4@15165 16af8721-9629-0410-8352-f15c8da7e697

examples/osg2cpp/osg2cpp.cpp

@@ -85,7 +85,7 @@ int main( int argc, char **argv )
     std::string filename;
     if (arguments.read("--shader",filename))
     {
-        osg::ref_ptr<osg::Shader> shader = osgDB::readShaderFile(filename);
+        osg::ref_ptr<osg::Shader> shader = osgDB::readRefShaderFile(filename);
         if (shader.valid())
         {
             std::string name = osgDB::getStrippedName(filename);