New Cairo operator: STRAIGHT

#include <cairo.h>

int main (int argc, char *argv[])
{

    float alpha=0.65;
    float alpha_circle=0.8;

    cairo_surface_t *surface;
    cairo_t* cr;
    surface = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, 480, 270);
    cr=cairo_create(surface);

    // Draw Circle
    cairo_save(cr);
    cairo_set_source_rgba(cr, 1.0, 0.5, 0.5, 1.0);
    cairo_move_to(cr, 140.0+70.0, 117.0);
    cairo_arc(cr, 140.0, 117.0, 70.0, 0.0, 2*3.14159);
    cairo_clip(cr);
    cairo_paint_with_alpha(cr, alpha_circle);
    cairo_restore(cr);

    //Prepare rectangle
    cairo_save(cr);
    cairo_set_source_rgba(cr, 0.26, 1.0, 1.0, 1.0);
    cairo_rectangle(cr, 164.0, 114.0, 186.0, 103.0);
    cairo_clip(cr);

    // this section of code performs the following operator:
    // aR = aA*alpha + (1-alpha)*aB
    // xR = (xaA*alpha + (1-alpha)*xaB)/xR
    // When alpha =0.0 aR = aB and xR = xB
    // When alpha =1.0 aR = aA and xR = xA
    {
        cairo_save(cr);

        cairo_surface_t* dest=cairo_get_target(cr);
        cairo_surface_flush(dest);
        cairo_surface_t* destimage=cairo_surface_map_to_image(dest, NULL);
        int w=cairo_image_surface_get_width(destimage);
        int h=cairo_image_surface_get_height(destimage);
        cairo_surface_t* newdest=cairo_image_surface_create(CAIRO_FORMAT_ARGB32, w, h);

        cairo_t* destcr=cairo_create(newdest);
        cairo_set_source_surface(destcr, destimage, 0, 0);
        cairo_paint_with_alpha(destcr, 1.0-alpha);
        cairo_destroy(destcr);

        destcr=cairo_create(destimage);
        cairo_set_source_surface(destcr, newdest, 0, 0);
        cairo_set_operator(destcr, CAIRO_OPERATOR_SOURCE);
        cairo_paint(destcr);
        cairo_destroy(destcr);

        cairo_surface_unmap_image(dest, destimage);
        cairo_surface_mark_dirty(dest);

        cairo_surface_destroy(newdest);
        cairo_restore(cr);
    }
    //

    // Effectively draw the retangle
    cairo_set_operator(cr, CAIRO_OPERATOR_ADD);
    cairo_paint_with_alpha(cr, alpha);

    cairo_restore(cr);
    cairo_destroy(cr);

    cairo_surface_write_to_png(surface, "straight-cairo-full.png");
    cairo_surface_destroy(surface);

    return 0;
}