Fixed "invalid ID" for darken terrain

[MatheusPese]

Fixed "invalid ID" for darken by adding a new terrain data for the "darken" tile

[Byteron]

I'll just copy here what I wrote in discord

"darken" was the tile used to get the black bars over non reachable tiles.
The shader we use now however does not use the darken tile. Instead, it uses the void tile.
Same goes for the fog shader.
The "darken" sprite and tres file was deleted.
I must have missed one place where it still says 'darken' somewhere.

I had a look, and there is still a "darken" tile in res://graphics/tilesets/terrain.tres.
Best would be to remove the darken tile from the tileset, as the editor creates it's buttons based on the tiles existing in this tileset.

[MatheusPese]

Oh, yeah, i thought of removing it, but then i thought that you where maybe needing it somehow.
I could maybe have made 2 different pull requests at the time, because both could be a possible solution for the problem.