Returns a GridCellSetter that can mutate array values based on cell {x,y} positions.

const arr = [
1,2,3,
4,5,6
]
const a = setMutate(arr, 3);
a(10, {x:0,y:0});
a(20, {x:2, y:2});

// Arr is now:
// [
// 10, 2, 3,
// 4, 5, 20
// ]