Some drawing operations... gdk_draw_layout(), gdk_draw_pixbuf(),
require a colormap, because either they may be done using
RENDER or they simply need to interpret colors.

Currently, we don't have a colormap for bitmaps, unless the screen's
has a real depth-1 visual, so it is impossible to use these functions
on bitmaps.

So, we should add a fake colormap/visual for bitmaps that has
0 = black, 1 = white, and a function:

 gdk_colormap_get_bitmap ()

That retrieves it. We might want to set this colormap automatically
on bitmaps created in contexts where a real colormap won't
be set on them.

[ This relates to the longer term plan of creating
  fake colormaps/visuals for all RENDER picture formats ]