perf: replace unstable Tile widget key with dedicated stable TileKey

[braindamagedman]

Previously tiles were keyed with ObjectKey(tileRenderer), but TileRenderer instances are recreated on every build. Because of that, the key was unstable even when the logical tile did not change. The root cause of this issue was introduced in commit
182c4fc when unbounded horizontal scroll was introduced.

As a result, Flutter could not match the new widget with the existing element and treated the tile as a new widget. This led to repeated disposal and recreation of _TileState instead of updating the existing state.

The fix replaces that key with a stable TileKey based on TileImage identity and positionCoordinates. This preserves _TileState for the same rendered tile and avoids unnecessary remounting.

Before:

After fix:

You can check simple code to reproduce before/after examples here:
https://github.com/braindamagedman/flutter_map/tree/test/tile-key

[JaffaKetchup]

Oo, thanks for this, we should've caught that. That's a useful debug text, we might add that to the example app.

We need to make sure that this isn't preventing the tiles from changing when we actually do want the state to change, since we've had far too many issues there in the past, but I think your fix should be good.

[JaffaKetchup]

Ok, LGTM, thanks! I am ofc hesitant in these parts but I think the change itself looks sensible and I can't see any issues with testing. Indeed, it looks like it has improved the performance a bit!

(I don't know why 100% we couldn't have just changed the equality of the tileRenderer, but I think this makes sense anyway.)