function getZoomPointsCenter(p, p1, p2) {
  p.x = (p1.x + p2.x) / 2;
  p.y = (p1.y + p2.y) / 2;
  return p;
}
Get center point between two points

@param {Point} p
@param {Point} p1
@param {Point} p2
@returns {Point}