Instead of draining only into air/non-walkable below, allow
draining into any space air can pass, including ones where the
prills would be immediately displaced, e.g. into a frame.
This also allows it to drain directly into an existing stack of
glowing prills without an intervening airspace. This should tone
down the difficulty of figuring out ore smelting a bit, and
make for somewhat more interesting lode furnace builds.
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