Simple ECS demo including the following:

3 components:

• Position -> Store a x/y position
• Velocity -> Store a x/y velocity
• Density -> Store a weight

2 system:

• Gravity -> Add 9.8 * Density.weight to the y velocity
• Movement -> Add the velocity to the position

4 entities with different expected behavior:

• One with only a position -> shouldn't move
• One with a position and a density -> shouldn't move
• One with a position and a velocity -> should move following velocity
• One with position, velocity and density -> should move with an increasing velocity