Support multiple brushes per tile in TileMap

[Shadowblitz16]

Describe the project you are working on

A mario game which I will provide a editor where users can draw there own graphics.

Describe the problem or limitation you are having in your project

I need to be able to reuse tiles in brushes.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

I suggested a feature similar to this before called combos, which was based on a app called zquest's combo system.

Basically combo's are a list of tile index group with other properties that tell the tile what it is, how it acts, and how it looks, how it collides, and how it animates.

The difference between Godot's current system and the combo system is combos are not directly 1:1 tile to combo ratio, instead combo are their own separate list.

In zquest you can only paint with combos but I don't see why godot's couldn't paint with tiles also.
The only difference is they wouldn't support things like physics and animation since they would only be index into the tileset.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Basically I am suggesting something similar where we can define more then 1 brush per tile.
Right now brushes overlap tiles so you can't put another brush over a tile which already has a brush on it.

My suggestion is to not tie the data with the tile and instead just have these brushes be their own separate list.

If this enhancement will not be used often, can it be worked around with a few lines of script?

It would be used alot since people usually don't like editing their tilesets to add another brush for a tile that looks the same.

Is there a reason why this should be core and not an add-on in the asset library?

Tileset is already part of core and it just makes things easier to work with.