Collisions: Report X of a collision

Reporting collision is useful for sound. X coordinate
can be used to position the sound.
SOUND! Make "knock" on collision

A better sound would be welcome.
Do not require collision in exact pixel

Check if objects overlap to calculate collision. This allows us
to use almost any speed in the game (*) without tunneling.

* Any speed = speed < Thikness of the thinnest object to collision.