Write access to underground pipe/belt drag status

[sparr]

I'd like to be able to, mid-drag, change which entity the game thinks was placed previously for the purposes of figuring out where to place the next underground pipe or belt.

See viewtopic.php?t=129192 for context.

[boskid]

Given that this is a piece of logic that is still likely to see changes in terms of implementation, adding any API for it is risky because then it would become bounding to keep API functional which would limit the possible changes in the drag logic. This request is unlikely to get implemented at this moment.