Overlay terrains

[mog]

We have overlay terrains for some time and I'd like to discuss which terrains could/should be converted to overlays.

I also just committed a change, which makes it possible to define a default base for overlay terrains. This means that such a terrain will behave like a normal terrain with (default_base^overlay) in the editor, unless you paint while holding ALT. That's useful e.g. for forest as it would be annoying having to paint over a region twice (once for base, once for overlay). Default base terrains have no effect on gameplay.

So, on to the list:

Already converted:

• Wooden bridges
  alias: base, flat
  No default base
• Lit terrain
  alias: base
  No default base
• Sand rubble
  def. alias: base, hills
  mvt. alias: worst of base, hills
  No default base
• various forests/ great tree:
  aliases: same (i.e. base is just decoration)
• farmland
  alias: base

Suggestions:
Generally set the default base so that it won't change the current editor behaviour. I noted exceptions.

• Desert crater (needs graphics changes)
  current alias: sand
  proposed alias: sand
• Oasis (needs graphics changes)
  current alias: sand
  prop. alias: sand
• windmill
  current alias: flat
  prop. alias: base
  no default base
  This one should really be changed into a village of some kind.

Villages:
Those are a bit of a problem. Currently, flat villages are aliased to the village terrain, while most other villages are best of village and base (Merman villages are aliased to their base, but that makes sense).
If we allow both kinds of villages to be placed on arbitrary base terrains with those aliases, we create horrible inconsitencies (villages on the same base terrain have different defence/movment), except maybe for merman villages as those are different enough from other villages.

If we change the aliases to one of the two types this might have a noticable gameplay effect on current maps, which is probably not desirable.

I suggest changing the mereman villages to aliasof=base.
I'm not sure about other villages. I'd like to have some variety at least for the flat villages, but can we rely on the mapmakers not to make stupid things?

Hm... I just had an idea: what about an "allowed_bases" property for overlay terrains? So we could restrict placing flat villages on flat terrains only. Would be easy to implement, too.

[mog]

I converted forests, rubble and farmland. Crater and oasis are not yet done due to my lazyness.

I'd still like opinions about windmill and village terrains, though.