set_difference_blender - Enables a difference blender mode. Allegro
game programming library.
#include <allegro.h>
void set_difference_blender(int r, int g, int b, int a);
Enables a difference blender mode for combining translucent or lit
truecolor pixels. This makes an image which has colours calculated by
the difference between the source and destination colours.
set_trans_blender(3alleg), drawing_mode(3alleg)