Fixed RenderGunboat wake animation

There was a white pixel in the wake animation that has bugged me for quite some time. Upon closer inspection, it turned out that the white pixel was standing still because the wake animation was only played once instead of looping.

The white stands out in an annoying way even on the looping animation though, so I fixed the art as well.

Just watch the shellmap or play the first GDI mission with and without this, you should see the difference.