Gigas improvements

 * Attack poison gaz OK (still a little messy when hero is going down, but
I'm not sure I will fix that)
 * Attack summon has been changed to a summon of bee guard (less powerful,
but the general pattern of Gigas is already complicated to not add
difficulty)
 * Preparation for Thunder Strike (it will be a pain in the ass)...
Sprite for thunder strike almost done
First version (broken) of thunder strike

Added the attack to Gigas with a new animation (hands up infinite).
Bug : the thunder strike is not going to the good direction, the
superposition is a mess. Probably a misuse of the sprite system.