Please add flag color for non-generated chunks

[Poikilos]

Having a special flag color for non-generated chunks (such as #010000 with alpha 0 if drawalpha is enable which I had working in my fork of the python version), would be very helpful for map viewer software that use images from minetestmapper, especially map viewers that generate multiple images. This would distinguish non-generated chunks (and edge chunks, but I don't remember whether they are actually stored differently than non-generated chunks in the map format or if I just imagined that) from:

• pixels generated from air blocks within the given y range (no change to current behavior, if uses the background color with alpha 0 if alpha enabled)
• pixels generated from nodes where the colors.txt has no corresponding color (which should probably have some kind of innocuous flag color like #898888 and be fully opaque)