I think your approach is correct and I don't see any problem with the code you posted. (given that the custom methods you call work as they are supposed to ) What are the values of fwidth and fheight? They should be the size of the texture and power of two, but I guess you know this allready!

If you flip the buffer after rendering the scene the first time (for the cctv-texture) you should see the complete scene on your screen but in the size of the texture in the low left corner. does it work?

and my last question: how do you generate the blank texture?

greetings,
-lith