When image_map'ing a PNG texture with alpha channel to a surface, that surface
still casts a fully opaque shadow.
See <web.4a00bbfb2b1505395dd6915e0@news.povray.org> for a sample shot; it shows
a semi-transparent PNG image of a leaf, UV-mapped to a mesh2 square floating
above a grey plane, with light from straight above.
As can be seen, the shadow doesn't match the shape of the leaf.
