Flashing potions (WIP)

Starting to fix #1796

Can currently test on purple & green potions. Player will flash potion colour while potion is in effect.  I'm not entirely certain what happens to the timer when you either select another potion or get hurt. It should cancel the timer & start flashing the new colour but I'm not confident about that.

To get potions from the command line, use "-x give_potions" or from the cheat screen "no no juice".