In order to guarantee that a slice never exceeds its parent,
EdgedPlaneSlice was introduced by #1082.

However, it added complexity, and happens to be insufficient to adapt
get_intra_edges() for tiling (we can't retrieve the tile dimensions).

Instead, pass both the Plane (in the future, the plane region associated
with the tile) and the plane offset. It's simpler, and it still does not
require a slice to exceed its parent.