Make RenderUnit.PlayCustomAnimBackwards not crash if the callback is null
Fix cargo loading/unloading.
- Fix the unloading subcell bug, letting us have units move to directly adjacent cells.
- Have the cursor change to a deploy-blocked cursor if the transport can't unload due to terrain type.
- Add RenderTransport for transport door opening.
- Remove turning/opening in general.
Update rules for cargo changes
Update CHANGELOG for cargo changes
Add map upgrade code for UnloadFacing removal
Merge pull request #4616 from ScottNZ/transport

Fix cargo loading/unloading