Set mask with helper function in Entity's constructor
Add some tests for Mask

* Fix an issue with the callback called from collide and not passing
  mask along.
* Fix the issue of the collision happening at the bounds:
    {x, y, width, height}
    {0, 0, 1, 1} and {1, 1, 1, 1} are colliding at point (1, 1)
  This now returns true on such collisions.