mktrans | Make background transparent using antialiased alpha channel

The -p option is ugly and probably nobody wants to use it. Maybe I should remove it just to make the documentation shorter and clearer. Also, nearly every single bug listed here is due to -p existing. Running this script on an image that already has transparency will erode the image due to the antialiasing. Using -A is a workaround, but is not very satisfactory. Perhaps this script should remove any existing transparency before manipulating the image and then add it back in at the end. But then again, how often are people going to want to do that? The only use I can think of is when using -p. Because of the previous bug, if you do use -p to fill lots of lagoons, you'll probably want to use -A at the same time. Finding the coordinates for -p is a pain. It'd be nice if there was a -P option which let the user click on a point (or multiple points) in the image to start the floodfill.