Added stats collection to new Texture Pool code, and improved TexturePool implementation.

The Texture Pool can be enabled by setting the env var OSG_TEXTURE_POOL_SIZE=size_in_bytes.
Note, setting a size of 1 will result in the TexturePool allocating the minimum number of
textures it can without having to reuse TextureObjects from within the same frame.

src/osg/Texture2D.cpp

@@ -137,6 +137,10 @@ void Texture2D::apply(State& state) const
     // current OpenGL context.
     const unsigned int contextID = state.getContextID();
 
+    Texture::TextureObjectManager* tom = Texture::getTextureObjectManager(contextID);
+    ElapsedTime elapsedTime(&(tom->getApplyTime()));
+    tom->getNumberApplied()++;
+
     // get the texture object for the current contextID.
     TextureObject* textureObject = getTextureObject(contextID);