These values can be passed to
gdk_pixbuf_render_to_drawable_alpha() to control how the alpha
channel of an image should be handled. This function can create a
bilevel clipping mask (black and white) and use it while painting
the image. In the future, when the X Window System gets an alpha
channel extension, it will be possible to do full alpha
compositing onto arbitrary drawables. For now both cases fall
back to a bilevel clipping mask.