FR: Allow Beams to keep on target from a moving source.

Right now, if a beam source moves, the beam will not stay on the original target, but also move in the same way.
Add a flag to beams that offsets their target by the negative movement of the beam source , as to keep the beams hitting the same spot even if the beam source moves.
This is presumably a prerequisite to make #3516 work when the beam source is moving as well.