Added new buffered_value template class which encapsulates a std::vector but

initializes the array to the number of graphics contexts, and automatically
expands the array when indices outside the current size are required.

Added new osg::Texture::Extensions nested class to handle extensions on a per
context basis.

src/osg/Texture2D.cpp

@@ -68,18 +68,7 @@ int Texture2D::compare(const StateAttribute& sa) const
 void Texture2D::setImage(Image* image)
 {
     // delete old texture objects.
-    for(TextureNameList::iterator itr=_handleList.begin();
-                               itr!=_handleList.end();
-                               ++itr)
-    {
-        if (*itr != 0)
-        {
-            // contact global texture object handler to delete texture objects
-            // in appropriate context.
-            // glDeleteTextures( 1L, (const GLuint *)itr );
-            *itr = 0;
-        }
-    }
+    dirtyTextureObject();
 
     _image = image;
 }