Once we run the placement algorithm and resolve all the placement locations, we have something that looks like this:
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Each placement with a non-zero starting_pixel count has reserved those pixels on the biome grid. Now we begin expanding each of these placements until they either
1. Reach their desired total_pixels size; or
2. Become boxed in by other biomes and have nowhere left to expand to.
This expansion process happens in rounds, with each placement claiming one pixel per round.
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