0.14.21 regenerate_entities and miners

[rbtcollins]

I had a map that I made a littttle too sparse on resources, so I ran
game.regenerate_entity("iron-ore") to workaround that.

However, all the miners that had stopped due to having no ore, which now had ore under them, did not actually start mining again until I picked them up and put them down.

[Smarty]

Not a Bug

That's a side effect of an optimization done for mining drills and will be staying that way.

Once the drill runs out of things to mine it is shut off so it doesn't consume CPU time needlessly on the extremely low chance that ore is spawned under it.

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