Used by the routines at MOVE_GRENADE and ANT_TURN.

Frames $F4-$F7, chosen by the low two bits of the count. The grenade's count has already gone from 5 to 4 when this runs (MOVE_OBJECT), so its blast shows $F4 -- its flight frame -- then $F7, $F6, $F5. An ant's is written over by MOVE_ANT on every frame the ant moves, so the blast shows only on frames it skips: a slow ant flickers between ant and blast, and the fast one never shows it (measured in a simulator).