Radius Enchantment Mana Drain Equation Fixed

New equation is simpler and avoids any truncation issues so that mana is
drained correctly for all values.

Previously, Jala songs and radius enchantments drained the full mana cost
per drain time period. For example, Rejuvenate drained 8 mana every 5
seconds. The drain is now always 1 mana at a time, so Rejuvenate will
drain 1 mana every 0.625 seconds. It's visually much easier to track now,
since players don't always have 5 seconds to stare at their mana.