Bitmap where the bilevel mask will be painted to.
Source X coordinate.
source Y coordinate.
Destination X coordinate.
Destination Y coordinate.
Width of region to threshold, or -1 to use pixbuf width
Height of region to threshold, or -1 to use pixbuf height
Opacity values below this will be painted as zero; all other values will be painted as one.
Takes the opacity values in a rectangular portion of a pixbuf and thresholds them to produce a bi-level alpha mask that can be used as a clipping mask for a drawable.