For 'flash' targets only.
Iso-functions for MarchingSquares must be given
as an object implementing this IsoFunction interface. This is for
reasons of avoiding excessive memory allocations that occur through
automatic boxing of arguments/return values when using function values.
Since iso-functions may be called 10,000's of times per-invocation of
marching-squares, this can quickly accumulate into a lot of GC activity.