Control the alpha channel for drawables.
These values can be passed to gdk_pixbuf_xlib_render_to_drawable_alpha() in gdk-pixbuf-xlib 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.
There is no user of GdkPixbufAlphaMode in GdkPixbuf, and the Xlib utility functions have been split out to their own library, gdk-pixbuf-xlib.
A bilevel clipping mask (black and white) will be created and used to draw the image. Pixels below 0.5 opacity will be considered fully transparent, and all others will be considered fully opaque.
For now falls back to #GDK_PIXBUF_ALPHA_BILEVEL. In the future it will do full alpha compositing.