collision free hasing for ChunkID

The "quick way to hash a pair of integers" can also generate collisions.
As it is a simple XOR for X and Z coordinate, several collisions are
generated with typical coordinates. Some examples:
x=0 z=1 -> 1 / x=1 z=0 -> 1
x=2 z=0 -> 2 / x=0 z=2 -> 2 / x=6 z=4 -> 2
...
At my tests Qt was adding some seed, so it worked anyway. But it should
be safer to use non-colliding hashes so it will work even when Qt
changes the internal implementation.