The row-major index of (x, y): its position in a flat width * height array.
Deliberately unchecked. Call cellInside when the coordinates are not already known to be
valid; an out-of-range x otherwise lands on a neighbouring row rather than failing, and a
negative one on the row before it.
The row-major index of
(x, y): its position in a flatwidth * heightarray.Deliberately unchecked. Call
cellInsidewhen the coordinates are not already known to be valid; an out-of-rangexotherwise lands on a neighbouring row rather than failing, and a negative one on the row before it.