Implement operator[] for PlaneMutSlice
Implement operator[] for PlaneMutSlice

Implement traits Index and IndexMut to access PlaneMutSlice rows as if
it was a two-dimensional array.

Thay way, we can write:

    plane_slice[8][8] = 1;
    let row = &mut plane_slice[5];
    row[3] = 42;

xiph/rav1e,xiph/rav1e