intLayer
% A |PointLayer| is used in the first stage of boundary determination. In
% the APEX/application, it is the mechanism for implementing clicking and
% erasing the initial version of the boundary. An |image| object with CData
% and AlphaData both set to the current state of the layer (the points
% which are selected) is used, as is a 2D matrix called the "stack", which
% records the order in which points have been clicked. This mechanism is
% important for the interpolation process, which is detailed