class explosion

The explosion class is tailored to give a good solution for explosions.

Explosion sprite is changing it's size by nature (explosion expanding), but it's center is constant.

The x,y parameters are the center of explosion , and the i_low and i_high are sprite index for explosion image.

The f_speed parameter tells the explosion object the frames-interval between each image (that is how fast explosion is changing).

Once explosion is done, it is automatically removed from dest BITMAP and the object sprite_index is set to -1.
Thus if(explosion_object->get_sprite_index() == -1) it's time to dispose the explosion object.

For each frame user have to call the remove(), action() and draw() method.