Interface which represents an Operation which is "drawable", such
as a Rectangle, Circle or Text. The Operation is responsible for
creating its own image buffer and drawing itself into it, then
wrapping and returning it as a PlanarImage. This allows multible
"drawable" objects to be nested.
Abstract method which is intended to create an image buffer
and return it so it can be drawn into another object. Use
an Alpha channel for a "transparent" background.