Adjust mob chance to cast for movement upgrades.

Shakrune's movement grid upgrades shorten the time between Chase Timer checks and thus between attempts at monster spellcasting. Time decrease is average 4x for slow mobs and 7x for fast ones, so in the
spellcasting chance calculation I multiplied piSpellChance by 6.

Adjusted the order in which conditions are checked in MonsterCastSpell to reduce unneeded calls to SquaredDistanceTo.