Delta's return inversed value

[GregHib]

val tile = Tile(100, 100)
val other = Tile(105, 106)
val delta = tile.delta(other)
assertEquals(Delta(5, 6), delta)

The delta from tile1 -> tile2 is returning the negative delta instead of the positive. That is to say x1-x2 not x2-x1.

Will need to flip all existing usages

[GregHib]

There's no reason it has to be one way or another, might it make more sense the other way though? Hard to say