More natural biome borders. (Terrain height in feature draw order?)

[will-ca]

I'm not suggesting we add edge-smoothing, texture splatting, or anything so radical to the Unciv core.

But a lot is already almost possible with the existing systems.

Proof of concept. Smooth, blended biome edge version of 5Hex:

Mask:

Proof of concept. Fractal biome edge version of 5Hex:

Mask:

These work by making the tile textures go slightly past the hexagon, so there's always an overlapping region where a gradual or fractal transition breaks up the hard edges.

The only problem is that doing it this way makes rivers and terrain features get covered up:

Possible solution: Take terrain height into account for ordering terrain features?

(It would also be useful to have a ~10% margin off to the side of each hexagon texture, but that would pose bigger compatibility concerns.)

[will-ca]

Alternate solution found within existing capabilities: Tileset can shift entire entire river image up to northwards of the covered/transition area.

[AdityaMH]

And how to add this mask for tileset mods?