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/trunk@15164 16af8721-9629-0410-8352-f15c8da7e697

examples/osgwidgetscrolled/osgwidgetscrolled.cpp

@@ -22,9 +22,9 @@ bool scrollWindow(osgWidget::Event& ev) {
     osgWidget::Window::EmbeddedWindow* ew =
         dynamic_cast<osgWidget::Window::EmbeddedWindow*>(frame->getEmbeddedWindow())
     ;
-        
+
     if(!ew) return false;
-    
+
     // Lets get the visible area so that we can use it to make sure our scrolling action
     // is necessary in the first place.
     const osgWidget::Quad& va = ew->getWindow()->getVisibleArea();
@@ -34,7 +34,7 @@ bool scrollWindow(osgWidget::Event& ev) {
     if(ev.getWindowManager()->isMouseScrollingUp() && va[1] != 0.0f)
         ew->getWindow()->addVisibleArea(0, -20)
     ;
-    
+
     else if(va[1] <= (ew->getWindow()->getHeight() - ew->getHeight()))
         ew->getWindow()->addVisibleArea(0, 20)
     ;
@@ -88,10 +88,10 @@ int main(int argc, char** argv) {
         osgWidget::WindowManager::WM_PICK_DEBUG
         //osgWidget::WindowManager::WM_NO_INVERT_Y
     );
-    
+
     osgWidget::Frame* frame = osgWidget::Frame::createSimpleFrameFromTheme(
         "frame",
-        osgDB::readImageFile("osgWidget/theme.png"),
+        osgDB::readRefImageFile("osgWidget/theme.png"),
         40.0f,
         40.0f,
 	osgWidget::Frame::FRAME_ALL