Dumping material next to a wall does not always completely fill in

Issue description

When dumping material next to a retaining wall, it does not always fill in all the way to the wall, leaving occasional divots that cannot be built upon.

Reproduction steps

1. Load a new sandbox or the attached save
2. Place walls
3. Mark a dumping zone that runs up to the wall
4. Wait for dump trucks to fill in the material until dumping is complete
5. Observe dips against the wall every 6 tiles or so

Game version and build number

v0.8.7d (b619)

Operating system

Windows