void al_clear_depth_buffer(float z)
Clear the depth buffer (confined by the clipping rectangle) to the given value. A depth buffer is only available if it was requested with al_set_new_display_option(3alleg5) and the requirement could be met by the al_create_display(3alleg5) call creating the current display. Operations involving the depth buffer are also affected by al_set_render_state(3alleg5).
For example, if ALLEGRO_DEPTH_FUNCTION is set to ALLEGRO_RENDER_LESS then depth buffer value of 1 represents infinite distance, and thus is a good value to use when clearing the depth buffer.