[AD] correct behaviour for additive blending?

[Peter Wang <novalazy@xxxxxxxxxx>]

Am I doing something wrong or is this a bug?

Here I draw two intersecting lines with additive blending.
On the offscreen bitmap the colour is as I expect (g=50; g=100 at the
intersection).  On the backbuffer the colour is only half as much (g=25;
g=50 at the intersection).

~~~~
#include "allegro5/allegro5.h"
#include "allegro5/allegro_primitives.h"

int main()
{
    ALLEGRO_DISPLAY *display;
    ALLEGRO_BITMAP *bmp;
    ALLEGRO_EVENT_QUEUE *queue;

    al_init();

    display = al_create_display(640, 480);
    al_clear_to_color(al_map_rgb(0, 0, 0));

    al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ONE, al_map_rgba_f(1,1,1,1));
    al_draw_line(100, 150, 200, 150, al_map_rgb(0, 50, 0), 0);
    al_draw_line(150, 100, 150, 200, al_map_rgb(0, 50, 0), 0);

    bmp = al_create_bitmap(100, 100);
    al_set_target_bitmap(bmp);
    al_clear_to_color(al_map_rgb(0, 0, 0));
    al_draw_line(0, 50, 100, 50, al_map_rgb(0, 50, 0), 0);
    al_draw_line(50, 0, 50, 100, al_map_rgb(0, 50, 0), 0);

    al_set_target_bitmap(al_get_backbuffer());
    al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ZERO, al_map_rgba_f(1,1,1,1));
    al_draw_bitmap(bmp, 0, 0, 0);

    al_flip_display();

    queue = al_create_event_queue();
    al_register_event_source(queue, al_get_display_event_source(display));
    while (1) {
        ALLEGRO_EVENT event;
        al_wait_for_event(queue, &event);
        if (event.type == ALLEGRO_EVENT_DISPLAY_CLOSE)
            break;
    }

    return 0;
}