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/Demos/osgtexture3D/osgtexture3D.cpp

@@ -49,7 +49,7 @@ class ConstructStateCallback : public osg::NodeCallback
             }
 
             // get max 3D texture size
-            int textureSize;
+            GLint textureSize;
             glGetIntegerv(GL_MAX_3D_TEXTURE_SIZE, &textureSize);
             if (textureSize > 256)
                 textureSize = 256;