Extract position-setting into a callback function

[shippy]

The reason position isn't set in config is that for many objects, their position is a function of (a) screen width and height, (b) other objects. For those, however, the computation is deterministic.

settings.object.X.posFn could be the standard place for an object to save the requisite function handle in.

The standard format is function XPos(ScreenWidth, ScreenHeight [, settings.objects]) -> rect, where rect is a 4x1 matrix. If the format is kept constant, this will enable other functions to extract other object positions without necessarily knowing what their specific position-calculating function is, simply by calling settings.object.X.posFn.

[shippy]

offCenterByPx can handle a lot; users can make their draw scripts a function of other things. This is too much complexity for too little help.