Big changes!

Entity: Changed texture loading detection.
Font: Now uses string instead of a char-array. Uses new bulk rendering
in the spritesheet.
Input: Now takes in and returns proper values for mouse position.
Main: No longer sleeps thread if drawing takes more than 1ms.
Rect: Added method for intersection without pointers.
ScreenTest: Is now a demo scene for lights and shadows, with intructions
displayed on screen.
SpriteSheet: Now has methods for "bulk" rendering to minimize glBegin()
calls, greatly increases performance.
AiralWhite16: A white version of the font, for the test screen.