fix: apply same delta from position to previous position when setParts runs

[tylerfurtwangler]

Problem

Currently when setParts is run, it sets the position and positionPrev to the new total parts centre values. This is fine when the body is stable/constant when being set, however for actively moving objects this will cause the new body to have no delta between positions.

This factors in when calculating the new velocity during update: https://github.com/tylerfurtwangler/matter-js/blob/master/src/body/Body.js#L638

Fix

This change uses the existing body positionPrev and position values to come up with a delta to apply to the body to keep the previous relative positions the same during a setParts call.