Fix off-by one error in automatic layout

The coordinates of diagram ports were rounded to the next grid position,
while all other positions (bend points, component ports) were rounded
down. As a result, the position of diagram ports can sometimes be off
by one grid position. This probably was not noticed because the size
of diagram ports in the diagram tape measure was multiplied by 2, so
that the issue happend less often.

Example:
File > Open AF3 Example > Load ACC Example, open component
AdaptiveCruiseControl and do automatic layout. Some arrows that
should be horizontal are not quite.

Signed-off-by: Ulrich Schöpp <info@ulrichschoepp.de>
Issue-Ref: 4184