Migration: Map reverts tiles to hidden_tile, if exists

[Mylon]

TL;DR, the game knows what green-concrete SHOULD be if the mod is removed for whatever reason, so use that information.

Specifically, when a mod that adds custom placed tiles is removed, the tile reverts to grass-1, but the map still retains the hidden-tile information. This hidden-tile information can create some inconsistent behavior (where the existence of a hidden-tile usually only happens because the tile was placed). On migration, the game could revert these to their hidden_tile and remove the hidden_tile state.