The point of this function is to produce two ranges x1..=x2 and y1..=y2
that are guaranteed to be safe to iterate over the canvas of size
canvas_width by canvas_height without any boundary checks.
The point of this function is to produce two ranges left_x..=right_x and
top_y..=bottom_y that are guaranteed to be safe to iterate over the
canvas of size width by height without any boundary checks.