Add Advanced Modifier

[BenjaTK]

AdvancedModifier(2D/3D) is a modular modifier which will allow for much more complex generation.

It works with an array of conditions, all of which it have to be met to place the tile on the target cell.

For example:
Let's say we want to place vegetation on top of side-scrolling terrain. We'll use an OffsetCondition, which checks for a certain tile in an offset to allow placing.

In this case, the first condition is only met if there is a ground tile below.
We also have a chance condition, which is pretty self explanatory.

Now we'll make the tile to be placed a RandomTileInfo, which will choose between trees, grass and tall grass.

Here's the result:

We can also do stuff like this (the torches):

This PR also adds a filter type to only apply modifiers on empty tiles.

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To-do

• OffsetRule3D
  
• Documentation
• Icons for the modifier and rules

[BenjaTK]

I am also not entirely sure about the wording of "rules"? Any ideas?