That was a tough task. The points outside of the outline of the cage
are not defined in green coordinates, so we would either need to:

- chop the grid into smaller chunks near the outline, which would make
  the grid non-uniform with lots of inherited complications

- extrapolate the points near the outline of the cage based on the
  internal points which lay inside the cage polygon.

I chose the latter approach. It is much easier to implement, but has one
small drawback. Sometimes, if your cage is too narrow (e.g. 8-16px wide),
the interpolator may not find the points suitable for a base. In such a case
the corresponding cell of the grind will be dropped from the processing
and you may see an empty cell.

The cell sizes:

1) Preview: 16 px
2) Real transform: 8 px

So when creating the cage, just ensure you are not creating the cage
polygons more narrow than 8 px. This limitation applies only to the
non-deformed grid. The deformed one can have arbitrary configuration.

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