Struct-valued custom enumerator for pixel offsets

[Lehonti]

I think one shouldn't have to allocate on the heap in order to enumerate the pixel coordinates and offsets.

To be consumed by a foreach loop an object doesn't need to implement IEnumerable<T>, but only expose public methods with the corresponding names and signatures.

It is my understanding that this new approach would prevent heap allocations except when the object is explicitly used as an IEnumerable<T> (like in VoronoiDiagramEffect through an invocation of Select).

[cameronwhite]

Could you try running the effect benchmarks to see if there's a notable difference with this change? If so I'm okay with it, but otherwise the original implementation is much simpler