Only center production line boxes if necessary

For lines with e.g. 2 input resources and 2 output resources, start at
0 instead of 1 each (as would have happened before). Calculating center_y
however is always required for aligning the toggle icon (cogs) correctly.

All or most of this is irrelevant anyways, as right now there simply do
not exist production lines which would have exposed the faulty behavior.