allow filling of closed shapes

[Mark Lodato]

It would be nice to be able to fill shapes with colors. For example, the flow chart from the documentation could color "Start" and "End" green, "Init" and "Process" blue, and "more?" yellow. A reasonable interface would be via the command line: the user gives text/color pairs. For each pair, the program colors any closed shape that contains that text. So, the above example would be done via the command-line arguments `--flood=88ff88:Start --flood=88ff88:End --flood=8888ff:Init` and so on.

[zsquareplusc (zsquareplusc)]

Providing the arguments on the command line has drawbacks:
- there are several plugins that use aafigure in other environments
  that would need to be changed to allow these args too
- the colors would not be visible in the aafigure source image
  but in an other place

I could imagine a legend within the image itself.

The bigger issue is that shapes are currently not closed polygons, so that its not possible to simply define them as filled polygons in e.g. the SVG output... This would require some sort of optimizer pass that creates polygons from touching lines, then detect closed shapes. A bigger task...