void draw_target(BMAP*);
//...
draw_target(pRenderTarget); // Set a render target
draw_line(vector(x1,y1,0),NULL,100);
draw_line(vector(x2,y1,0),_vec(0,0,255),100);
draw_line(vector(x2,y2,0),_vec(0,0,255),100);
draw_line(vector(x1,y2,0),_vec(0,0,255),100);
draw_line(vector(x1,y1,0),_vec(0,0,255),100);
draw_target(NULL); // Reset render target