The WorldRenderer outlives the TerrainSpriteLayer and thus keeps it alive longer than expected via the event handler. We detach it to allow the GC to reclaim it.
PPos is best thought of as a cell grid applied in
screen space. Multiple cells with different
terrain heights may be projected to the same PPos,
or to multiple PPos if they do not align with the
screen grid.
PPos coordinates are used primarily for map edge
checks and shroud / visibility queries.
