fix for setPlanetRadius breaks setPosition

[csibbitt]

There is some unexpected interaction between setPlanetRadius() and setPosition().

The lua scenario code below works - every 10s the planet gets further from the origin and switches texture. However, if I uncomment the setPlanetRadius(), then the radius and texture update, but the position does not.

function init()
    P = Planet():setPosition(sectorToXY("F6")):setPlanetRadius(2500):setPlanetSurfaceTexture("planets/planet-1.png")
    player = PlayerSpaceship():setTemplate("Phobos M3P"):setPosition(0, 0):setRotation(0)
end

sectorNum = 6
lastupdate = 0
function update(delta)
    lastupdate = lastupdate + delta
    if lastupdate > 10 then
        sectorNum = sectorNum + 1
        P:setPosition(sectorToXY("F" .. sectorNum))
        P:setPlanetSurfaceTexture("planets/planet-" .. (sectorNum % 5) + 1 .. ".png")
        lastupdate = 0
    else
        --P:setPlanetRadius(sectorNum * 500)
    end
end

This patch fixes it. For some reason it was the first thing I tried, and it worked, but I can't yet describe why it fixes it; so not convinced it's the correct thing to do. With this patch, the radius, texture, and position all update as expected.