Replaced allocateMipmap code with warning message as mipmaps aren't support for TextureRectangle.   
Replaced allocateMipmap code with warning message as mipmaps aren't support for TextureRectangle.   


git-svn-id: 23b6355f2bb369032457de4eb5f713ee134f73a8@7392 16af8721-9629-0410-8352-f15c8da7e697

joevandyk/osg,joevandyk/osg,joevandyk/osg,joevandyk/osg,joevandyk/osg