Limit undo/redo to only backup modified maps

[TokisanGames]

Currently undo/redo retains everything, which is fine for ~10 regions. I haven't noticed any issues, but surely, we'll run into problems with speed or memory consumption when working with much larger terrains.

We don't have a per region dirty flag. Currently it's map type only.

There might be a challenge when creating regions in the middle of painting or at least it was a challenge before when getting us to this point.

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Update since f626dc1: Undo/redo now only retains info for the current tool. No longer all map types for all regions, now it is just height maps for sculpting tools, control maps for texture tools.

What is remaining is limiting undo/redo to regions modified, which should be addressed by, or after #374. Perhaps the editing brush can be continually adding regions to the snapshot as the brush goes over them.