Parallax background images

Got parallax background layers working. Everything set up from the game level script. In this image there's one "space" 1000x1000 distance units (light-seconds?) backdrop image with a parallax factor of 0.9, meaning it pans at x0.9 the speed of the foreground.

Also there's fire, smoke and sparks coming out of that engine... :-)

I'll soon have to stop adding more stuff in and look at optimising the rendering performance as it's all very raw at the moment and I know there's lots to optimise away.