Destructive skills' effects remain visible after backwards frameskips

[Nami Verasche]

Reproduce:

1. Assign a basher, miner or digger and let it remove some terrain
2. Do a backwards frameskip or instant restart
3. Removed terrain is still removed visually (but it is replaced functionally)

[Nami Verasche]

Temporary kludge-fix in 28a2828.

[Nami Verasche]

A permanent fix is still needed, but since a working kludge fix has been implemented, I can lower the priority of this for now.

[Nami Verasche]

I believe this has now been properly fixed; if not, I'll get to it during general code tidying.