Scale postprocessing

**This one is hard!**

The algorithm doesn’t divide into neat tiles. Pixel values can “spread” from tile to tile.

Naive / inefficient algorithm:
- Repeat until convergence:
- For each pixel:
- Spread pixel values to neighbors as controlled by the pixel mask.

We could do something similar, processing each tile until it converges. If a tile pushes to its neighbors, those would need to be reprocessed as well.

Something like:
- Enqueue every tile
- Repeat until queue is empty:
- For each tile:
- Spread pixel values within the tile.
- If the pixel values push into other tiles: put those tiles back on the processing queue.