Made it easier (I hope) to avoid getting killed twice in a row.

Made it easier (I hope) to avoid getting killed twice in a row.
We now take the square root of the ship's velocity, instead of just half.
This appears to put it nicely in the middle of the hole left by the
explosion, whatever speed you are going.

Also, you now have control over your ship during the DEAD_PAUSE; so it fades
in, and is invulnerable until it becomes fully opaque.  Note that the dead
pause is still the same length as it was before.