Terrain rocks can be dug from any height

Issue description

Specifically, the rocks placed on the terrain

I had an excavator dig one several layers below it's height down/across a cliff. Maybe this is intended though?

Reproduction steps

Try to place mining designation over a rock at a a height at which it should be invalid, says valid, and excavators will try to dig it

Game version and build number

Update 1 0.5.3d (224)

Operating system

Windows

Reminds me of another issue that was created about this a berZillion game years ago.