1. Updated the Score to XP ratio: XP earned after game = 1/10 of the
player's score

2. Implemented the Overdrive skill.
When the player presses the Square button, Overdrive is activated and
boosts the ship's speed. Activation of the skill depletes the Energy bar
by a rate of 50% per second.
The speed boost is relative to the number of skill points invested:
Lv1-Lv5: 1.5/2.0/2.5/3.0/3.5 x base speed

3. Updated the Energy cost of the Dematerialize skill:
Lv1-Lv5: 100/90/80/70/60 % of maximum Energy per second