Visual spell damage types

When taking damage, if the damage has an elemental component, the
player's screen will now flash a color representing the element instead
of just red pain.

* Holy damage comes with an ethereal white flash.

* Unholy damage makes you momentarily see black & white.

* Acid causes a deep green flash.

* Shock causes a light blue flash.

* Fire causes a light orange flash.

* Cold causes a deep blue flash.

* Quake damage causes a brown flash.

This should help players more quickly assess the kinds of damage they're
taking, without having to read fast-moving text messages mid-combat.
As always, duration of the flash depends on damage done. These flashes
are half as long as EFFECT_PAIN visual durations.

Note: since they now have shock colors done for them, shocking fury and
lightning bolt no longer do EFFECT_INVERT, which was also causing
players to see through blind.

Also note: these flashes cannot be turned off the same way that Show Pain can
be turned off.