I thought RDP was already a bit more intelligent than "only" sending raster frames, sending things like window dimensions and such to be rendered on the client?
Try remoting into a machine over RDP and playing a youtube video occupying a large portion of the screen. You may be surprised to find that it plays nearly perfectly, even over non-ideal network conditions (wifi). Try this same exercise with VNC and you will experience a frame maybe once every other second.
I am not sure exactly the heuristics involved, but RDP is certainly switching between modes of operation based on what kind of visual information is on the screen.