Expose separate rows from PlaneSlice

[rom1v]

Currently, many functions access rectangular regions of planes (spanning multiple rows) via a primitive slice to the whole plane along with the stride information.

This strategy is not compatible with tiling, since this borrows the memory belonging to other tiles.

This is basically what I explained in #631 (comment).

Therefore, only expose separate rows in PlaneSlice.

Concretely, here are the changes from the caller point of view:

// get a specific row

// before
let row = plane_slice.as_slice()[y * stride..(y + 1) * stride];
// or if we don't care about the end
let row = plane_slice.as_slice()[y * stride..];

// after
let row = &plane_slice[y];

// get a component value

// before
let value = plane_slice.as_slice()[y * stride + x];

// after
let value = plane_slice[y][x];

// iterate over rows

// before
for row in plane_slice.as_slice().chunks(stride).take(height) {
    let _first_four_values = &row[..4];
}

// after
for row in plane_slice.rows_iter() {
    let _first_four_values = &row[..4];
}

Note that in itself, it does not solve the incompatibility with tiles (PlaneSlice itself still borrows the whole plane primitive slice). This is only the first step to remove all usages of raw slice + stride information, so that we can then replace PlaneSlice by a tile-specific "plane region" structure which does not borrow memory outside the tile (cf #821).

For now, I only submit changes for PlaneSlice. I will submit similar changes for PlaneMutSlice once I get feedbacks on this one (and when it's finished).

EDIT: #1043 does the same for PlaneMutSlice

[lu-zero]

The two unsafe usages could be wrapped in a different way later, the rest looks pretty neat already :)

[coveralls]

Coverage increased (+0.2%) to 82.026% when pulling 993af98 on rom1v:planeslice.23 into 7fd3938 on xiph:master.

[tdaede]

This looks excellent!

[rom1v]

Oops, parentheses missing:

let remaining = self.ps.plane.cfg.height as isize - (self.ps.y + self.next_row as isize);

It's fixed in my PlaneMutSlice pull request #1043.

[tdaede]

Weird that it didn't get picked up in tests.