Fade subtiles based on distance to null tiles

[sisby-folk]

The map looks silly when zoomed out:

And it's partially because textures don't tile with the outer unexplored area, making them look like a brick of disconnected mess.

If we make textures tile to unexplored areas, this happens:

The hard square border looks ugly, and it looks even uglier on hard-tiling textures like dense forests.

If texture alpha was gradually lowered towards an unexplored area, the map might appear to have "fog", which would allow us to take the second approach without the square downsides.

We can't change the way the tile is drawn due to batching, so this should be done by redrawing the background over top of the subtiles at various alpha levels (subtiles grouped based on some algorithm for this, then batched per-opacity)

[sisby-folk]

here's a vague mockup of that - we can probably do better in code