Dude, if you REALLY must use this really slow technique for your gfx. I suggest you never use CLS and only plot pixels in parts of the screen where there are tanks or movement underneath, then you're never drawing the entire screen each frame. That'd be faster, but still not really that fast and also you'd need some pretty good algorithms and lists to sort out your drawing routines.
Except you can't actually *count* on the backbuffer not getting cleared by the driver, so there is no guarantee that you still see the 'unaffected' areas on the screen.
In addition to that, some video adapters alternate between multiple backbuffers, meaning you need to account for all movement during the last *couple* of frames if you choose not to redraw the entire screen... Not just the last one.