crop for Model

[christo]

Crop would truncate geometry outside a given rect, cut would do the same but remove geometry inside the given rect. All isolated points to be cut would simply be removed.

Lines that bisect the boundary need to be split with new endpoints on the boundary and would result in additional elements if, say, a Polyline enters and exits the cut region.

For Asteroids this must be extended to allow toroidal space where an object can be partly on one edge of the screen and partly on the opposite edge. For corners this could be a four-way split. Until then, make an off-screen margin large enough to contain the largest asteroid so that the modulus coordinates don't force interpolation lines between points at opposite extremes of the Model space.