Alien hunter leap ability

Alien hunters can now leap, Hitting a mob stuns it, Hitting a dense object stuns the alien for a short time.
Uses WJ's glorious Alien leaping sprites.

(Hud code a shit.)

Uses a modified throw_at() that doesn't restrict movement to cardinal dirs, this is so the alien doesn't look fucking retarded when it moves on diagonals.

Aliens in "leap mode" cannot attack, this is intentional.