We add a dirt-like particle (drawn from scratch, uses some
colors from default_dirt's palette) to spawn many particles
that have collision enabled around the center of the blast.
This has the effect of obscuring the center of the blast, as
that is a painfully visible empty area when the explosion happens,
as there's only a little spark.
The dirt particles bounce around the walls and floor a bit,
and disappear rapidly, well before the smoke particles disappear.
This is a nice visual distraction that obscures the sudden
appearance of the gaping hole, and makes it a whole lot more
believable.
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