When displaying polygons on a BalloonCanvas, setting the alpha of the
fill-color apparently does not work.
It changes the color though: when alpha approaches 1.0 the original
color shows, when alpha approaches 0.0 the resulting color approaches
black/opaque. Is this a known limitation, or a bug.... (the code looks
like transparency should be handled). The final drawing is done in
primitives, so a bit difficult to check..

Hans