(greymap->bitmap greymap)
A greymap is converted to a new bitmap, each grey value >= 128 is counted as white. For bitmaps, a bit value of 0 is white and a value of 1 is black.
The resulting new bitmap is returned.
> (greymap->bitmap (greymap-create 600 1200))
#<bitmap orow:-1 ocol:-1 xres:-1 yres:-1 h:600 w:1200>
>