When real spiceblooms explode, they spawn an invisible
spicebloom.spawnpoint actor, which only 'grows' when the cell is of
GrowthTerrainType. When 'exploding', it then displays the animation of
blowing spice up into the air, and spawns a real spicebloom.
This replicates the original behavior where spice blooms would only grow
on sand cells that had been harvested and free of spice.
Improve seeding target cell determination.
Draw out spice seedable cells before loop.
Default to null for SpiceBloom.Weapon.
Pull spice seeding into its own method.
Skip it completely if no seed weapon is defined.
