Normalize Tile Palette Empty Source Tiles

[ilexp]

Summary

If there are multiple empty tiles in a Tileset, all of them can be used for painting tilemaps. While there is no visual distinction, flood-fill and runtime operations will still identify them as different tiles. This is a potential pitfall that can be removed on the editor side by normalizing tile palette selection.

Analysis

• Similar to how the Tile Palette normalized painted AutoTiles to the base tile, it could normalize painted empty tiles to the default empty tile.
• Tilesets should be extended to expose a default empty tile index, which can be used for this.

[ilexp]

Done by @cowmanjoe in PR #761.